I installed Firefox 3.0 Beta 4 (V. 3.0b4) last week. Again, the improvement
focused on security and UI. I couldn’t “feel” improvement on performance though.
I decided to uninstall it yesterday and found some problems after removing it.
I’d like to share my experience with you so you can save some time figuring out
the problems if you happen to uninstall Firefox 3.

I got an error message after clicking a link in Microsoft Outlook saying
“This operation has been canceled due to restrictions in effect on this
computer. Please contact your system administrator.”

I fixed the problem by going to Microsoft Internet Explorer 7 (MS IE7) and set
it to be the default browser, and then set Mozilla Firefox 2 to be the default
browser. Problem fixed!

If you only have one browser available, you can go to Start -> Control Panel ->
Add Remove Programs -> Program Access and Defaults -> Custom -> Choose a default
Web browser -> choose whatever is not selected -> click OK -> choose the default
browser you prefer -> your problem is fixed.

 Hope this helps!