Try the further options listed here - http://kb.mozillazine.org/Default_browser

I tried every tip I found, and still am unable to make firefox my default browser. In fact, I am unable to make any browser my default! I also use IE, Chrome, Opera. Opera used to work as default, and I tried to switch back to Firefox. I uninstalled Opera, and Firefox still would not switch over. I uninstalled Firefox, and reinstalled it, still nothing. When I check to see if Firefox is my default, it says it is. Can you think of anything else? Thanks.

Try using this program.
http://windowsxp.mvps.org/defaultbrowser.htm

I tried this program. Unfortunately, It did not set firefox as my default. If you know of any other solutions, please let me know. Thank you, paul

If you recently downloaded Opera 11.11 and it took Firefox (or whatever your default was) here's how to get Firefox back and still keep Opera.

This worked for me!

http://www.guidingtech.com/6579/change-default-browser-make-firefox-chrome-ie-default-browser/

Good Luck all.

You can also do a system restore point in your control panel if all else fails.

To do this (XP ONLY) go to Start>Control Panel>Performance and Maintenance

In the upper left hand corner you'll seen ' System Restore" click that then click restore my computer at a earlier time then click next

Look for a very recent date (day before) then follow the instructions. Once it restarts, you sign in (if password protected) then you you'll see you have your default browser back again.

Downfall to this is - it removes all programs you installed that day BEFORE you restore. So if you download a few programs BEFORE you restored all you have to do is download them again. If you made registry changes or system changes you'll have to redo them. Very simple things.

If you've done nothing nor added any programs then you'll be fine just use the step-by-step above.

This is only if all else fails. Truly it's much faster than uninstalling and reinstalling all the programs like paul 6387 mentioned. Why wasting all your valuable time on reinstalling and uninstalling when it takes 5-6 minutes to fix the problem this way.

This is a suggestion the choice is yours if you chose the fastest way.

(if you do not have system restore or you chose to turn off system restore then you have to use what I wrote in the earlier post. )

Wish you luck : )