All of my Firefox add-ons have been disabled suddenly, how can I re-enable them?

Solution 0.5: For those that don't want to update

As of 2019-05-14, Mozilla's blog lists extensions that can be installed for users of older versions of Firefox who don't want to update (emphasis added):

• If you are running Firefox versions 61 – 65 and 1) did not receive the deployed fix and 2) do not want to update to the current version (which includes the permanent fix): Install this extension to resolve the expired security certificate issue and re-enable extensions and themes.
• If you are running Firefox versions 57 – 60: Install this extension to resolve the expired security certificate issue and re-enable extensions and themes.
• If you are running Firefox versions 47 – 56: install this extension to resolve the expired security certificate issue and re-enable extensions and themes.