There's definitely something wrong if it's so hard to go to 64-bit. This has been their problem for almost a decade.
(This holds for most performance-critical, memory-access-bound software.)
1) The OS X and Linux versions of Firefox have generally been slower than the Windows version all along, even before switching to 64-bit. Largely because gcc (and later clang/llvm on Mac) generated slower code at least for this particular codebase than MSVC did. This is also why the Windows release versions of Firefox are in fact compiled with MSVC, not gcc.
2) Most benchmarking done by the press is done on Windows, and most of the users are on Windows. Performance regressions on Windows do in fact matter more, both in terms of actual user experience and marketing/perception than performance regressions on other platforms. I wish that were not so, but it is.