You are aware that both issues are being worked on?
Beta versions are available as 64-bit. Release 64bit builds are available on ftp.mozilla.org, but not offered on the main download page yet because of unresolved bugs.
> Compound that with all tabs running in the same process
That's something a lot of development effort is going into right now. Available in the nightlies if you want to help test.
(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.
There's no doubt reasons for this that Mozilla can't fully control, but users have nonetheless voted with their feet.
It would be ideal of Mozilla was able to keep up, but the issue of Mozilla's declining share seems to have less to do with what Google is doing and more about what Mozilla is not doing.
Edit: I certainly don't mind if my opinion is in the minority, but please don't down vote simply because you disagree. That tool is supposed to discourage off-topic and "ad hominem" attacks, not differences of opinion. If I'm incorrect in my assessment, tell me why. =)