This seems odd but if Wikipedia is correct, you are better off with Firefox 3.6. Firefox 3.6 is shown as still being supported whereas Firefox 4.0 -7.0.1 are shown as end of life whih means you will not get security fixes for them.

As for one thing that 3.6 will not do but the current version will do, svg graphics (at least with as in img - I'm unsure about the other methods) comes to mind. It is one of those browsers that I have to provide what I (and I would imagine some other sites also) regard as legacy support for in the form of png graphics.