This is the new name for Phoenix, a web browser which used the Gecko rendering engine and the XUL toolkit, as used by Mozilla. MozillaFirebird? has recently become pretty popular and is currently slowly becoming the replacement for the Mozilla Browser component of !SeaMonkey?, the Mozilla application suite.
The latest release is 0.6, the first with the Firebird name.
See also:
No other page links to MozillaFirebird yet.