FreeBSD is moving to the CLANG/LLVM compiler. Firefox currently fails to build using CLANG, complaining that "MMX instruction set not enabled". CLANG seems to treat MMX and SSE separately, and MMX must be explicitly added in CFLAGS. See the following link for more details:
Fix: Apply the following patch, if it makes sense. I'm not good with conditional makefiles.
Patch attached with submission follows:
How-To-Repeat: Add the following to /etc/make.conf:
Attempt to build www/firefox (it doesn't work).
Over to maintainer (via the GNATS Auto Assign Tool)
Should be fixed in the latest version