I successfully installed Firefox 3 on WinXPPro-SP2 this evening. I already had FF2. Despite a clean install, I was unable to actually launch it. I got an immediate crash and the subsequent crash detector happily sent my crash report to Mozilla. After the 3rd time I gave up.
I uninstalled it and tried again. No change. So I downloaded and installed 126.96.36.199 and I'm back up. I suspect now that the problem might lie with FireBug and its compatibility with FF3.
All I was really trying to do was reinstall with the DOM inspector so I could look at style sheet behavior. Fortunately, that is now working. I was also happy to see that all my settings, customizations and add-ons were seamlessly restored. Kudos to Mozilla (at least for pre version 3).