Hello everyone!
I have Avast Online Security & Privacy installed and I currently use the latest version of Firefox. The extension keeps disappearing or uninstalling itself. I don't know why this happens, but when I reinstall it, it keeps every setting that I had modified intact. This has happened on both my Desktop computer(i have AVG antivirus installed) and on my laptop(i have Avast Free Antivirus installed) many times. I keep reinstalling and after some days or so it disappears again. Feel free to ask my for any more info if I missed to tell something.
Have a nice day/evening!