The project strategy was originally modeled after that of Mozilla's Firefox, and Thunderbird is an interface built on top of that Web browser.

[9] Thunderbird is an e-mail, newsgroup, news feed, and instant messaging client with personal information manager (PIM) functionality, inbuilt since version 78.0 and previously available from the Lightning calendar extension.

[10] With contributors all over the world, Thunderbird has been translated into more than 65 languages,[6] although email addresses are currently limited to ASCII local parts.

[18] Thunderbird provides mailbox format support using plugins, but this feature is not yet enabled due to related work in progress.

[19] The mailbox formats supported as of July 2014[update] are: Thunderbird also uses Mork and (since version 3) MozStorage (which is based on SQLite) for its internal database.

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

The delivery format auto-detect feature will send unformatted messages as plain text (controlled by a user preference).

It also offers inbuilt support for secure email with digital signing and message encryption through OpenPGP (using public and private keys) or S/MIME (using certificates).

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

[25] The software has a chat client for instant messaging, supporting the IRC, XMPP, and (since version 102) Matrix protocols,[26] and the Odnoklassniki network.

[40][41] 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.

[44] On September 17, 2007, the Mozilla Foundation announced the funding of a new internet communications initiative with David Ascher of ActiveState.

The purpose of this initiative was "to develop Internet communications software based on the Thunderbird product, code, and brand".

[49] On July 6, 2012, a confidential memo from Jb Piacentino, the Thunderbird Managing Director at Mozilla, was leaked and published to TechCrunch.

[53] On December 1, 2015, Mozilla Executive Chair Mitchell Baker announced in a company-wide memo that Thunderbird development needed to be uncoupled from Firefox.

[56] Therefore, at the same time, it was announced that Mozilla Foundation would provide at least a temporary legal and financial home for the Thunderbird project.

[61] The interim/beta versions Thunderbird 57 and 58, released in late 2017, began to make changes influenced by Firefox Quantum, including a new "Photon" user interface.

[68] Alongside the transition, OpenPGP support was integrated directly into Thunderbird as a standard feature, seeking to supplant the Enigmail extension.

[70] On January 28, 2020, the Mozilla Foundation announced that the project would henceforth be operating from a new wholly owned subsidiary, MZLA Technologies Corporation, in order to explore offering products and services that were not previously possible and to collect revenue through partnerships and non-charitable donations.

[75] Thunderbird 91 features various UI improvements (including a new account setup workflow), Apple silicon support, CardDAV address book support, built-in import and export tools for Thunderbird profiles, the PDF.js PDF viewer, and the ability to encrypt emails to BCC recipients.