Mozilla are not developing a version of Firefox for the Blackberry.

They are developing a Firefox Home app for the Blackberry which will let you synchronize your bookmarks with the Blackberry. I do not know when this will be releasedFor details on Firefox Home see http://www.mozilla.com/mobile/home/

A 3rd party Build is being developed by one of the Mozilla developers as a side job.

http://www.fractalbrew.com/labs/prefox/

Well let me tell you that Tony your answer really bites. Firefox needs to develop a browser for the blackberry. We are the customers and they should listen

I'm a bit late to this thread but I believe the reason behind this is the lack of a native development kit for BlackBerry (https://wiki.mozilla.org/Mobile/Platforms#Other_Platforms). With RIM's announcement of a NDK for the PlayBook maybe we'll see Firefox there eventually (http://press.rim.com/release.jsp?id=4935)