I don't think that I have any problems with Fx17 at all. It's not about Fx not working on 64-bit Windows, Mozilla halted development of Win64 builds of Firefox until further notice due to most likely incompatible plug-ins that don't work correctly in 64-bit browsers.
I LOVE Firefox 17.0.1. Appearence-wise, it hasn't changed, but performance-wise, oh yes. It's actually not as slow as Firefox 16 was (Kept not responding and kept slowing down at times), so I really have to applaud the creators for speeding it up!!!!!
I reluctantly updated today, from Firefox 15 to 17.01... reluctantly, because of the dumb change with the tab-bar and the lost of my favorite ''theme'' (incompatible with v.17)... and now, I seem to be facing a weird bug that I can't solve OR find information about.
So... I found a new theme for Firefox 17, installed it and restarted Firefox. Marvellous, it works fine.
Then, I close Firefox (to do something for a few minutes), reopen it, and... my ''theme'' is reseted to 'Default' AND my 'TACO' addon is deactivated even though it's compatible (If I click to update it, it say ''ready to update'', restart Firefox, work fine (even though the version doesn't change)... then is ALSO reseted to 'Deactivated' if I close/reopen Firefox).
It's rather annoying... and could be problematic to me.
Windows 8 is almost a month old and its expected to be widely adopted for the next three years. Mozilla drops support for older platforms only due to the significant technical challenges as outlined above.
10.5 support was removed in Firefox 17 as it was technically infeasible to port the new features back to 10.5 and the costs of keeping support for 10.5 wound up being too high.