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.