I've compiled the latest Firefox source (nightly) with various gcc compiler optimization options, compared to the "default" moz build options on my platform. This is not a comprehensive test, but merely gives some ideas of where the "best bang for the buck" is. I ran the Sunspider benchmarks three times for each build instance. My box is a single CPU core2 system with 4GB RAM. The compiler options used were as follow:
-Os -march=core2 -mtune=core2 -O2 -march=core2 -mtune=core2 -O3 -march=core2 -mtune=core2 -O2
The last is the default optimization options on my platform, which is what I call the "Moz" build. The "core2" options turns out to have little effect (1.01x - 1.03x at the most), but "-O3" vs "-O2" has significant impact ("-O3" always generates the fastest browser on my platform). And of course, the new JIT compiler is very fast. For now, all I have is this little screen shot of a table I made with the results, I'll convert this to a proper HTML table in my copious spare time.
The way to read this table is a little bit complicated, but pick a row (e.g. "O3 + JIT") and follow it to the column you wish to compare it to (e.g. Moz + JIT), and you see that the "O3 + JIT" build is 1.10x faster. The conclusions I can make is that it's always best to optimize as much as possible, optimizing for size doesn't makes sense on these modern CPUs (core2), for the Firefox code base. And JIT is always vastly superior.
As a side note, the build with the "-Os" options (optimize for size) was incredibly unstable with the JIT compiler enabled. It took me many, many runs to get it to complete three benchmarks without hanging.