Assuming that your computer has sufficient memory (and hard drive space) you should always use the latest version of any browser which is compatible with your operating system.
That means that you ought to be using Firefox 10.0.02 if you're using Windows 2000, XP, Vista or 7. (It's MUCH faster than Firefox 3.6!).
If you'd rather use Internet Explorer you should be using IE9 as long as you've got Windows Vista or 7. (IE8 is the last version which is compatible with XP).
I wouldn't uninstall Firefox. (I'd simply update it). But the instructions are here:
http://support.mozill...ninstalling%20Firefox
Chris