Hello, this morning I noticed that the Avast Online Security icon wasn’t showing up in my Firefox add-ons, so I uninstalled and reinstalled it by clicking “Remove” on the add-ons list, and then I reinstalled it by going to the add-ons store, but now the add-on won’t work at all, and when I click on it, the site rating won’t show up. Please can you help me fix this?
It’s okay. I fixed the problem by restarting my laptop.
Restart when in doubt. 
I have found over the years that ''rebooting" the OS has cleared up many anomolies that seem to have popped up out of nowhere.
Good advice “restart when in doubt”
Pretty much the first course of action as it does often produce results.
Its almost the first question asked if you call your companies tech support, have you switched it off and on again
The IT Crowd https://en.wikipedia.org/wiki/The_IT_Crowd and https://www.imdb.com/title/tt0487831/ I loved that TV program.
Very true. And the first question I ask, which as you say David “often produces results”.