Mr. Majestic's response is correct in general.
For Firefox you go to
http://www.mozilla.com/en-US/firefox/all-older.html
You download whichever one you want based upon your language preference, you unpack it, you run it.
Code:
$ tar jxvf firefox-3.5.11.tar.bz2
gives you a directory called firefox which contains, well, Firefox. Run with
or if you want to be a bit fancy and assuming you want the the US version you can do this which doesn't leave the firefox-3.5.11.tar.bz2 lying around.
Code:
$ curl -L "http://download.mozilla.org/?product=firefox-3.5.11&os=linux&lang=en-US" | tar jxf -
$ firefox/firefox
Note that last time I looked, Mozilla are dropping support for 3.5.X in August. Doesn't say when in August.