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Mozilla Thunderbird is a free and open-source[10] cross-platform email client, news client, RSS and chat client developed by the Mozilla Foundation. The project strategy was modeled after that of the Mozilla Firefox web browser. It is installed by default on Ubuntu desktop systems.

Mozilla Thunderbird
Mozilla Thunderbird logo.png
Mozilla Thunderbird 45.0 on Ubuntu
Mozilla Thunderbird 45.0 on Ubuntu
Developer(s) Mozilla Foundation (formerly Mozilla Messaging)
Initial release July 28, 2003; 14 years ago (2003-07-28)
Stable release 52.5.2 (December 22, 2017; 26 days ago (2017-12-22)[1]) [±]
Preview release 58.0 beta 3 (January 7, 2018; 10 days ago (2018-01-07)[2][3]) [±]
Repository Edit this at Wikidata
Written in C, C++, JavaScript,[4] CSS,[5][6] XUL, XBL
Operating system Windows XP SP3 or later; OS X 10.9 or later; Linux[7]
Size 33 MiB
Available in 53 languages
Type Email client, news client, feed reader
License MPL 2.0[9]

On December 7, 2004, version 1.0 was released, and received more than 500,000 downloads in its first three days of release, and 1,000,000 in 10 days.[11][12]

On July 6, 2012, Mozilla announced the company was dropping the priority of Thunderbird development because the continuous effort to extend Thunderbird's feature set was mostly fruitless. The new development model shifted to Mozilla offering only "Extended Support Releases", which deliver security and maintenance updates, while allowing the community to take over the development of new features.[13][14]

On December 1, 2015, Mozilla Executive Chair Mitchell Baker announced in a company-wide memo that Thunderbird development needs to be uncoupled from Firefox. She referred to Thunderbird developers spending large efforts responding to changes to Mozilla technologies, while Firefox was paying a tax to support Thunderbird development. She also said that she does not believe Thunderbird has the potential for "industry-wide impact" that Firefox does.[15][16] At the same time, it was announced that Mozilla Foundation will provide at least a temporary legal and financial home for the Thunderbird project.[17]



Thunderbird is an email, newsgroup, news feed, and chat (XMPP, IRC, Twitter) client. The vanilla version was not originally a personal information manager (PIM), although the Mozilla Lightning extension, which is now installed by default, adds PIM functionality. Additional features, if needed, are often available via other extensions.

Message managementEdit

Thunderbird can manage multiple email, newsgroup, and news feed accounts and supports multiple identities within accounts. Features such as quick search, saved search folders ("virtual folders"), advanced message filtering, message grouping, and labels help manage and find messages. On Linux-based systems, system mail (movemail) accounts are supported. Thunderbird provides basic support for system-specific new email notifications and can be extended with advanced notification support using an add-on.[18]

Junk filteringEdit

Thunderbird incorporates a Bayesian spam filter, a whitelist based on the included address book, and can also understand classifications by server-based filters such as SpamAssassin.[19]

Extensions and themesEdit

Extensions allow the addition of features through the installation of XPInstall modules (known as "XPI" or "zippy" installation) via the add-ons website that also features an update functionality to update the extensions.

Thunderbird supports a variety of themes for changing its overall look and feel. These packages of CSS and image files can be downloaded via the add-ons website at Mozilla Add-ons.[20]

Standards supportEdit

Thunderbird follows industry standards.

For Email there is

  • POP. This enables you to move your mail from your remote email server to a local folder. IMAP does not.
  • IMAP. IMAP itself has many capabilities, and Thunderbird selected many of them, plus added their own and added the de facto standards by Google and Apple.[21]
  • LDAP address completion.
  • S/MIME This is native, but through add-ons such as Enigmail instead.

For web feeds (e.g. news aggregators), there is Atom and RSS.

Thunderbird follows internet protocols as they evolve. For example, As of 2017, they are on IRCv3.1 IRCv3 came out in 2016. For usenet news groups they offer the latest NNTPS. And so on. But see § Limits and known issues.

File formats supportedEdit

Thunderbird provides mailbox format support using plugins, but this feature is not yet enabled due to related work in progress.[22] The mailbox formats supported as of July 2014 are:

  • mbox – Unix mailbox format (one file holding many emails)
  • maildir – known as maildir-lite (one file per email). Note: not yet stable, Mozilla advise this format is still "too buggy for normal use"[23]

Thunderbird also uses Mork and (since version 3) MozStorage (which is based on SQLite) for its internal database. Mork was due to be replaced with MozStorage in Thunderbird 3.0,[24] but the 8.0 release still uses the Mork file format. The current version of SeaMonkey, version 2.14.1, also still uses Mork for its indexes for both POP and IMAP mail folders (at least).

Big file linkingEdit

Since version 38, Thunderbird has integrated support for automatic linking of large files instead of attaching them directly to the mail message.

HTML code insertionEdit

The Insert menu provides the ability to include HTML code (<sub>, &mdash;, etc.) into a message being composed.

Limits and known issuesEdit

Thunderbird has some email issues.

  • POP3 folders are subject to filesystem design limitations, unlike software that uses its own blob (computing). This is the well-known 32-bit limit for 4 GB files. Each folder is one such file. Data corruption possible if this size is exceeded,[23] although Thunderbird can detect and prevent this. The limit was being actively worked on by developers in 2013[25] and remains open;[needs update?]. It does not affect IMAP mail storage.[23] Compaction of near-4 GB mailboxes is slow.[23]
  • POP3 folders with long filenames are subject to local-machine filesystem limitations. By comparison, storing folder names on an external machine can allow nested email folders with arbitrary length paths and filenames.[23]
  • The ISO-8859-1 charset is not fully supported.[26][not in citation given]

Cross-platform supportEdit

Thunderbird runs on a variety of platforms. Releases available on the primary distribution site support the following operating systems:[27][28]

Unofficial ports are available for:

The source code is freely available and can be compiled to be run on a variety of other architectures and operating systems.

Internationalization and localizationEdit

With contributors all over the world,the client is translated into more than 50 different languages, although email addresses are currently limited to ASCII local parts.[33] Thunderbird does not yet support SMTPUTF8 (RFC 6531) or Email Address Internationalization.


Thunderbird provides enterprise and government-grade security features such as TLS/SSL connections to IMAP and SMTP servers. It also offers native support for S/MIME secure email (digital signing and message encryption using certificates). Any of these security features can take advantage of smartcards with the installation of additional extensions.

Other security features may be added through extensions. For instance, Enigmail offers PGP signing, encryption, and decryption.

Optional security protections also include disabling loading of remote images within messages, enabling only specific media types (sanitizer), and disabling JavaScript.

The French military uses Thunderbird and contributes to its security features, which are claimed to match the requirements for NATO's closed messaging system.[34]


Originally launched as Minotaur shortly after Phoenix (the original name for Mozilla Firefox), the project failed to gain momentum. With the success of the latter, however, demand increased for a mail client to go with it, and the work on Minotaur was revived under the new name of Thunderbird, and migrated to the new toolkit developed by the Firefox team.

Significant work on Thunderbird restarted with the announcement that from version 1.5 onward, the main Mozilla suite would be designed around separate applications using this new toolkit. This contrasts with the previous all-in-one approach, allowing users to mix and match the Mozilla applications with alternatives. The original Mozilla Suite continues to be developed as SeaMonkey.

On December 23, 2004, Project Lightning was announced which tightly integrated calendar functionality (scheduling, tasks, etc.) into Thunderbird. Lightning will support the full range of calendar mechanisms and protocols supported by the Mozilla Calendar infrastructure, just as with modern (post-0.2) Sunbird.

On October 11, 2006, Qualcomm and the Mozilla Foundation announced that "future versions of Eudora will be based upon the same technology platform as the open source Mozilla Thunderbird email program."[35] The project is code-named Penelope.

In late 2006, Debian rebranded Thunderbird as Icedove due to trademark and copyright reasons. This was the second product to be rebranded.[10][36]

On July 26, 2007, the Mozilla Foundation announced that Thunderbird would be developed by an independent organization, because the Mozilla Corporation (a subsidiary of the foundation) was focusing on Mozilla Firefox development.[37]

On September 17, 2007, the Mozilla Foundation announced the funding of a new internet communications initiative with Dr. David Ascher of ActiveState. The purpose of this initiative was "to develop Internet communications software based on the Thunderbird product, code, and brand".[38]

On February 19, 2008, Mozilla Messaging started operations as a subsidiary of the Mozilla Foundation responsible for the development of email and similar communications. Its initial focus was on the then upcoming version of Thunderbird 3. Alpha Preview releases of Thunderbird 3 were codenamed "Shredder".

On April 4, 2011, Mozilla Messaging was merged into the Mozilla Labs group of the Mozilla Foundation.[39]

On July 6, 2012, a confidential memo from Jb Piacentino, the Thunderbird Managing Director at Mozilla, was leaked and published to TechCrunch.[40] The memo indicated that Mozilla would be moving some of the team off the project and further development of new features would be left up to the community. The memo was slated for release on July 9, 2012. A subsequent article by the Executive Chair of Mozilla, Mitchell Baker, stated Mozilla's decision to make a transition of Thunderbird to a new release and governance model.[13][14]

On November 25, 2014, Kent James of the volunteer-led Thunderbird Council announced on the Thunderbird blog that active contributors to Thunderbird gathered at the Mozilla office in Toronto and discussed the future of the application. They have decided that more staff are required working full-time on Thunderbird so that the Thunderbird Team can release a stable and reliable product and make progress on features that have been frequently requested by the community.[41]

On December 1, 2015, Mitchell Baker suggested in a company-wide memo that Thunderbird should be uncoupled from Firefox's infrastructure.[15] She referred to Thunderbird as being a tax on Firefox and said that she does not believe Thunderbird has the potential for "industry-wide impact" that Firefox does.[42] Mozilla remains interested in having a role in Thunderbird, but is seeking more assistance to help with development.[43]

On May 9, 2017, Philipp Kewisch announced that the Mozilla Foundation will continue to serve as the legal and fiscal home for the Thunderbird project, but that Thunderbird will migrate off Mozilla Corporation infrastructure, separating the operational aspects of the project.[44]

The beta version of Thunderbird 58, released in December 2017, begins to make changes influenced by Firefox Quantum, including a new "Photon" user interface, Microsoft Outlook import support, and the deprecation of XUL-based legacy add-ons in favor of WebExtensions.[45][46]


Thunderbird development releases occur in three stages, called Beta, Earlybird, and Daily, which correspond to Firefox's Beta, Aurora, and Nightly stages. The release dates and Gecko versions are exactly the same as Firefox; for example, Firefox 7 and Thunderbird 7 were both released on September 27, 2011, and were both based on Gecko 7.0.



  1. ^ "Thunderbird — Notes (52.5.2) — Mozilla". 2017-12-22. Retrieved 2017-12-28. 
  2. ^ "Thunderbird — Beta Notes (58.0beta) — Mozilla". 2018-01-07. Retrieved 2018-01-10. 
  3. ^ "Download in your language". Retrieved 2018-01-10. 
  4. ^ "Firefox's addons are written in JavaScript". Rietta. Retrieved December 19, 2009. 
  5. ^ "Firefox uses an "html.css" stylesheet for default rendering styles". David Walsh. Retrieved December 19, 2009. 
  6. ^ "The Firefox addon, Stylish takes advantage of Firefox's CSS rendering to change the appearance of Firefox". Archived from the original on May 23, 2009. Retrieved December 19, 2009. 
  7. ^ "Thunderbird 52.0 System Requirements". Mozilla Messaging. Retrieved May 3, 2017. 
  8. ^ "Download a Thunderbird that SPEAKS YOUR LANGUAGE". Retrieved March 1, 2015. 
  9. ^ Mozilla Licensing Policies,, retrieved January 5, 2012 
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  12. ^ "thunderbird 1.0 reaches 1,000,000 downloads in just 10 days!". Asa Dotzler's Blog. December 18, 2004. Retrieved February 9, 2013. 
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  15. ^ a b Baker, Mitchell. "Mozilla Governance: Thunderbird, the future, mozilla-central and comm-central". Google Groups, Mozilla Governance. Retrieved January 2, 2016. 
  16. ^ Lunden, Ingrid. "Mozilla Wants To Split Off Its Thunderbird Email/Chat Client, Says Mitchell Baker Memo". TechCrunch. Retrieved December 4, 2015. 
  17. ^ Kent, James. "Thunderbird Active Daily Inquiries Surpass 10 Million!". The Mozilla Thunderbird Blog. Retrieved January 2, 2016. 
  18. ^ "GNotifier". Retrieved July 21, 2017. 
  19. ^ "Mozillazine Forums". Retrieved June 18, 2009. 
  20. ^ "Themes :: Add-ons for Thunderbird". 
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  22. ^ "402392 – Support other message storage formats. (prelude to pluggable mail stores)". Retrieved March 1, 2015. 
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  25. ^ "793865 – nsIMsgParseMailMsgState.envelopePos must be 64bits to support large mbox folders (eg. 4GB)". 
  26. ^ "wrong characters in mail 2016 (updated) - Thunderbird Support Forum - Mozilla Support". 
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  32. ^ "Ports/os2 – MozillaWiki". Retrieved November 13, 2013. 
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  35. ^ "QUALCOMM Launches Project in Collaboration with Mozilla Foundation to Develop Open Source Version of Eudora Email Program". October 11, 2006. Retrieved November 13, 2013. 
  36. ^ "Uses Mozilla Firefox trademark without permission – Debian Bug Tracker". Debian. Retrieved September 18, 2010. 
  37. ^ Claburn, Thomas (July 27, 2007). "Mozilla Gives Thunderbird E-Mail The Boot". Internet section. InformationWeek. Retrieved July 31, 2007. 
  38. ^ "Mozilla Launches Internet Mail and Communications Initiative". September 17, 2007. Retrieved June 18, 2009. 
  39. ^ Paul, Ryan (April 5, 2011). "Thunderbird returns to nest as Mozilla Messaging rejoins Mozilla". Ars Technica. Retrieved April 5, 2011. 
  40. ^ Sarah Perez (July 6, 2012). "So, That's It For Thunderbird". TechCrunch. Retrieved August 18, 2012. 
  41. ^ "Thunderbird Reorganizes at 2014 Toronto Summit". The Mozilla Thunderbird Blog. November 25, 2014. Retrieved February 1, 2015. 
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External linksEdit