[Edit] Firefox 66.0.4 has fixed the certificate issue.

For those having issues with Firefox add-ons not working properly at this time type (this temporary measure) in the address bar "about:debugging" this will open up the window for you to manually add the add-on yourself via "Load Temporary Add-on" button. Make sure when you go on the Firefox official add-on page to NOT click on it as you'd normally do but to right-click > "Save Link As" > It should save the XPI file to your computer > Now simply go back to the window Debug tab and install it yourself until Firefox fixes the issue and also note unless you have to don't close the browser as you may have to redo the steps above.

For those who want to use your already installed add-on if it's in the default go to the following folder, C:\Users\%USERNAME%\AppData\Roaming\Mozilla\Firefox\Profiles\[name].default\extensions and look for the .xpi files. The [name] is whatever the profile folder is named after (usually its random) so copy/paste up to "Profiles". Share this to any other Firefox user having the same issue.

Linux users: /home/[yourusername]/.mozilla/firefox/[name].default
First instance of the [] replace it with first your user name and the 2nd instance may be random so you will have to find out what the name is. If further issues see this: http://kb.mozillazine.org/Profile_folder_-_Firefox#Linux
Also the above link is useful for MAC users.

Caution! The new Firefox temp fix may be a chance for Mozilla to gather telemetry. If this is an issue with you stick to the above method.

By the way I love this add-on. It makes it easy on my eyes even during the day. Tho some sites (or video players) don't like the add-on that's what the flip of the switch is for. Very simple and easy.