There was an issue (on Windows for sure) with the Thunderbird 10 Daily build from 2011-10-17, where add-ons would not install. Dragging an add-on file into the add-ons manager or using the Install From File option would not work. Plus the add-ons manager was acting a little odd as seemed to be randomly disabling add-ons even though I had Add-on compatibility checking disabled. Updating (Help > About Daily) to today’s (2011-10-18) Thunderbird 10 Daily should solve this issue.
