GAMER VET says:

Open Internet Explorer, and browse to http://www.mozilla.com
Click on the box near the top that says “Download Firefox – Free”.
Go through installation process — at the end, it will have a checkbox to “Launch Mozilla Firefox now”. Let it launch.
Once installed and launched, it should ask you if you want to make Firefox your default browser (answer yes) and if you want Firefox to check each time it starts to make sure it is the default browser (answer yes).
If you don’t get asked this:
- Run Firefox
- click Tools->Options
- select the “Main” icon in the popup box
- near the bottom, check the box that says “Always check to see if Firefox is the default browser on startup”
- Click the “Check Now” button next to that line, and answer “Yes” to the popup.
- Click “OK” to close the Options dialog and save your selections

J H says:

You don’t upgrade I.E. Firefox is a separate stand alone browser. Google for Firefox, the first link should lead you to their website, and you can download it from there.
Oh and you can’t uninstall I.E. Its a core program to Windows. You can use Firefox to change associations though, for specific types of things like HTML links, etc. On your first run of Firefox it should ask you if you want to do this. All it does is simply change which browser comes up when you click a link. First run of firefox will also ask you other things too, like importing your Favorites from I.E. etc. That way you don’t lose any of the settings you had from I.E. you’re just using them on a different browser platform.