I love Firefox and Thunderbird, but the Mozilla folks have been saying "you must uninstall before installing a new version" for too long now. Even as recently as 0.9.3 (the last version before 1.0PR), the release notes recommended this. What that says to me is that the installer might not get enough test exposure.
Having said all that, I still think it's reasonable to ask testers to uninstall/reinstall before reporting problems. BUT if the uninstall fixes the problem, it should still get reported, as it can highlight an underlying bug.