As with Firefox 1.01, uninstall Thunderbird 1 before installing the new version. Installing over the old version leaves the old version behind in add/remove and uninstalling the old version then breaks the new instalation.
Can we save the configurations, emails, account settings, etc. before the uninstalling?
How? Is it necessary or the new version won't overwrite them? If so, in a normal desinstallation, this files are just 'left behind'?