Extension developers frequently offer updates to their extensions. Thunderbird provides two ways to check whether your extensions have been updated. You can click the Update button in the Extension Manager or go to Tools | Options | Advanced and click the box under Software Update that is labeled Check Now to update your extensions and themes. If Thunderbird is unable to find any updates, it lets you know and provides a Details button you can click to see any errors that might have been encountered when the check was performed.
Upgrading to a New Version of Thunderbird?
If you have an older version of Thunderbird and you upgrade to a newer version, this check is done automatically upon installation. If Thunderbird finds extensions that are not compatible with the version you have installed, it identifies the extensions.