If one isn't found, you can override the compatibility check.Most add-ons will work if you do that, but it can cause the application to behave improperly.
Thunderbird will also disable the add-on and prevent you from enabling it by right clicking on the add-on in "Tools - Add-ons" and pressing the enable button.
The first thing to try is to check for updates to see if a new version is available.
(There is no way to set it back to false within the UI.) In other words, if you click on "Enable" you are resetting the extensions.check Compatibility.* preference to "true" and the flagged extensions will be disabled once more and not function.
To disable extension compatibility checking once again, you will have to re-create the preference and set it to false.
Note: The Add-ons Manager alert bar with the message Add-on compatibility checking is disabled. and "Enable" button (shown above) has been removed in future Mozilla applications (e.g., Firefox 4).