I just updated my Firefox to 66.0.3 on my Linux Mint 18.3 and all of a sudden, all my installed plugins were disabled (eg uBlock).
Turns out that the release was buggy and they are working on a fix. In the meantime, you can get things going again through a trick: by enabling “participating in studies” you can get access to their hotfix to remedy this problem (though it can take some time before you get notified once you enabled this option – up to 6 hours; but it only took a minute for me).
For more details and instructions, see the official Mozilla blog post:
https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/
If you haven’t updated to 66.0.3, then I’d strongly advice to wait for the next version to become available. Will save you some pain.