Thunderbird 7 disables some add-ons
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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.)
If you are unable to update Thunderbird for some reason, you can restore your add-ons with the simple workaround below: