Hi,
How about downloading source code from mercurial repositories and cross-compiling it for arm?
This might not be a good idea as to your requirements but it can be an option at the last.
You should place the source code into /opt/freescale/packages
and place the spec file into <ltib>/dist/lfs-5.1/FIREFOX/firefox.spec (or something like that)
Take a look on http://www.imxdev.org/wiki/index.php?title=All_Boards_Theora_Encoder
You can also use ./ltib -m shell go to firefox dir and use make or configure using the environment variable from LTIB