One of the cited reasons is that Linux version of Firefox is not built using PGO. PGO stands for Profile Guided Optimization. Windows version of Firefox is built on Visual C++ with PGO
IIRC Even Mozilla does not provide PGO compiled binaries for Linux. They have given a HOWTO for building Firefox with PGO, but how many of us can really do that?
If you want to get PGO compiled binaries, check Ted's PGO builds
OTOH Benchmarks show that even Windows Firefox over wine beats native Linux Firefox. If you are really unhappy, then use Chrome. Trust me, it is blazing fast.
There is a risk with PGO, since if the data provided is not proper, the performance of the final build is even worse than what it would be without PGO. Quoting from Wikipedia
The caveat, however, is that the
sample of data fed to the program
during the profiling stage must be
statistically representative of the
typical usage scenarios; otherwise,
profile-guided feedback has the
potential of harming the overall
performance of the final build instead
of improving it.