From within Firefox, click:
Tools > Options (then Options window will appear)
Click the "General" tab
Make sure the box is checked next to "Firefox should check to see if it is the defauld browser when starting."
Give it a test.
Some things, like Windows Update, will open in IE regardles of your preference.
I have used the Mac OS AND Window XP and unlike some other people, I actually prefer Windows XP.
The only problem I have is that I have set the mozilla firefox as my default web browser under the "set program access and default". Clicking on links within windows however will not load firefox. If I revert the default to internet explorer; clicking on the same link will load internet explorer and then the link site. Whats happening here? Is microsoft jerking me off?
System specs:
Windows XP with service pack 2 and lattest updates.
Internet Explorer 7 beta 2 preview.
Mozilla Firefox 1.5.0.1
1.3Mhz Clock
256mb SDRAM
Gateway MX3215
Thank for any suggestions.
By the way I welcome Apple mac users to the worm and virus club. 
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