Thunderbird 7 included a bug that caused some add-ons to be disabled and removed from the Add-on Manager. (Any data or settings associated with the add-ons are intact and will be restored once the add-on is re-enabled.)
We have released a Thunderbird update that fixes this issue. Please update Thunderbird or download and install the latest version from mozilla.org/thunderbird.
If you are unable to update Thunderbird for some reason, you can restore your add-ons with the simple workaround below:
Use the Add-on Recovery Tool (which works for both Firefox 7 and Thunderbird 7) to re-enable the add-on. The Firefox Help support site has an article with instructions on installing the add-on.
