Firefox 66.6.0.3 (its latest update) has developed an issue in which all extensions on a given user's copy of the app seem to have disappeared. The culprit, as Mozilla has noted, is due to an "expired intermediate certificate" which is necessary to authorize all of the add-ons in question.
This may be a considerable source of inconvenience to many users, particularly those who use reputable extensions that have worked trouble-free for some time. However, the open-source app-maker assures users that it will be resolved soon and their extensions will work as normal.
Then again, a number of individuals have responded with claims that this solution has not yet manifested itself on their devices. In the meantime, users have been advised not to take action to delete their respective add-ons, as this will carry the risk of losing all data associated with them.