When Thunderbird 78 launched earlier this year, it didn’t support automatic upgrades from version 68 or earlier. Automatic upgrade was blocked intentionally due to the revamped extension system that only supports MailExtensions, not classic extensions, to no break your Thunderbird installations. Three months later, Mozilla enabled automatic upgrades in Thunderbird with version 78.2.2 released a couple of weeks ago. Now Thunderbird 78.3 is out and it disables the installation of ???legacy??? MailExtensions.