Mozilla Firebird is a web browser and a spin-off project from Mozilla. The project aims to develop software that is smaller and faster than Mozilla by extracting and redesigning the browser part of the application suite. Mozilla Firebird retains the cross-platform nature of the original Mozilla, as it is written using the XUL user interface language. This contrasts with some other Mozilla-based projects — such as Galeon, Epiphany, K-Meleon, and Camino — as well as the popular Internet Explorer, which all use interfaces native to their respective platforms.
From version 1.6 of Mozilla onwards, Mozilla Firebird is expected to be coupled with Mozilla Thunderbird, a standalone mail client built on similar principles, to replace the current all-in-one application suite (codenamed SeaMonkey). This is expected to make Mozilla faster and more customizable. Therefore, after version 1.5 of Mozilla, Mozilla Firebird will be named "Mozilla Browser". However, this all was planned to happen one version earlier, so some are unsure that 1.6 will be based on Mozilla Firebird. Also, the application suite is expected to continue development after it stops being the main Mozilla distribution.
Mozilla Firebird version 0.7, codenamed Indio, was released on October 15, 2003. On October 26, 2003, Mozilla Firebird 0.7.1 was released for Mac OS X only. This point release contained a few bug fixes along with some performance tweaks.  Version 0.7 remains the current version for Microsoft Windows and GNU/Linux.
Mozilla Firebird was known as "Phoenix" until April 14, 2003, when a name change was made, because of trademark issues with Phoenix Technologies, a BIOS manufacturer. The new name — initially just "Firebird" without the "Mozilla" moniker — was met with a mixed reaction. Users and developers of the Firebird database server claimed that a Firebird web browser causes confusion. In late April 2003, the Mozilla Organization published a document stating that the browser should be referred to as "Mozilla Firebird" and not just "Firebird".