Does Add and Remove Programs show both a 3.0 entry and a 3.5 entry?
I have noticed that sometimes when I update Firefox, the new version is added to the list but the old version is not removed from the list. It seems that it is just a mistake in the update process.
My solution has been to use
CCleaner to remove the old entry. In CCleaner, go to "Tools" in the left column, go to "Uninstall", select the old Firefox entry, and click "Delete Entry". (If you decide to try CCleaner, I recommend going to
this page and downloading the "Slim" version.)