A new update for Thunderbird (7.0.1) that fixes add-on upgrades is now available, and all users are encouraged to update to the latest version from within the Thunderbird About window or by downloading from mozilla.org/thunderbird
We had identified an issue in which some users may have one or more of their add-ons hidden after upgrading to the latest Thunderbird version (7.0). These add-ons and their data are still intact and haven’t actually been removed. Released today, the latest update for Thunderbird fixes this issue. You can find more details on the Add-ons Blog.