Firefox 3.6 is actually still supported with security fixes, etc
No need for an etc.
I think it's safe to say you should expect to receive no more than major security fixes from such an old version. The nature of software development is that new features and enhancements are done in the trunk and branched out for new releases, with very little to no development effort put into back-porting the enhancements new features to older versions.
Safari for windows behaves completely differently from Safari for Mac
Sure it will, but it's not Apple's fault that Windows is based on DOS and OS X is based on BSD. There's only so much the browser developers can do given completely different software stack and APIs. I bet all the browsers have their little Mac/Windows idiosyncrasies. At least Apple are making an effort to develop for Windows unlike Microsoft and Internet Exploder (not that you would want it on Mac, but some websites written by incompetent browser blind developers only support IE).