Firefox 3.6 is actually still supported with security fixes, etc but for how long no one knows (I don't think it has been decided yet). I use a couple of add-ons that have not been brought into the newest versions and I have found issues with the newer versions on certain websites which aren't in 3.6.
I am employed working on software which uses web browsers as a front end. The short release cycle is a real problem for my company - we now have a team pretty much dedicated to fixing issues that appear in each new browser version (be it Chrome or Firefox, IE or Safari). It means I have everything installed... As I've found out, you can install both an old version of firefox (3.6) and the newest versions side by side - so long as you only run one of them at a time... Now if only the same piece of software would work the same on every operating system (Safari for windows behaves completely differently from Safari for Mac).

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.

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).

False

True

Well, whatever it's based on, my core point remains that they are chalk and cheese in terms of APIs etc.