Even on Windows you don't need to run the latest version, In fact, NT4 from 1996 (albeit with servicepacks from much later) runs Firefox 2 flawlessly. (just as an example.)

With Mac OS, if it's not oS X, it's classic (OS 7 to 9) and it's inherently at the end of its pretty much at the end of its useful-life-for-general-public, so you may as well use an old version to get the best performance.
_________________
[Insert Cool Signature here!]