While it appeared to many users that the add-ons and their data had disappeared during the upgrade, both were in fact still intact, Scott explained.

Firefox 7 users can download a free to restore their add-ons, but those who download Firefox 7.0.1 or update from within the Firefox About window won't have to worry about the problem.

In a separate issue, some users are finding their add-ons not just hidden but actually after updating Firefox, Mozilla noted. Those affected by that problem will see their add-ons listed as disabled in the browser's Add-ons Manager; guidance for fixing the problem can be found on Mozilla's site.

85 Percent of Users

There are some 9000 free, community-contributed add-ons available for Firefox. Mozilla recently that roughly 85 percent of Firefox users have installed, so maintaining a smooth transition across releases is particularly important.