Firefox has received TWO updates in the last week related to broken add-ons and themes and you should have 66.0.5 installed now.
If you don't have FF set to automatically update do so manually by clicking on the three lines (hamburger) icon and choosing Help and then About, which will trigger the updating (click the button to start it) and it will close and reopen the browser.
When you have problems like this it is often FF trying to tell you something, so heed it.
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I don't know nothing, and I memorized it in school and got this here paper I'm proud of to show it.