After two failed attempts last year, and a few glitches yesterday, we finally managed to get our Linux (and, obviously, Linux64) builds to use GCC 4.5, with aggressive optimization (-O3) and profile guided optimization enabled. This means we are finally using a more modern toolchain, opening opportunities for things such as static analysis. This also means we are now producing a faster Firefox, now much closer to the Windows builds on the same hardware on various performance tests.
A nice side effect of some of the work I have done to make the switch possible is that these builds will also work on older Linux platforms such as RedHat/CentOS 5, or possibly older (as long as they come with libstdc++ from GCC 4.1).
The first Firefox release to benefit these new settings should be Firefox 6.
A few branches other than mozilla-central have also been switched, most notably Try, for which there is a known issue if you push something too old. Please make sure to read the corresponding information on wiki.m.o for a workaround. A Mercurial hook is going to be put in place to issue a warning if there are chances your build will fail (it will, however, not prevent the push).
Thanks to Chris Atlee, Rail Aliiev, Taras Glek, Justin Lebar and all those I forgot or am not aware of for their assistance and/or past involvement in the previous attempts.
Looks like things will be better for us on GNU/Linux next time around. No need to settle for Chromium, FF fanboys!
There are some builds of FireFox versions out now that have PGO (Profile Guided Optimization) done by the community, but these will be straight from Mozilla themselves.