Glad I found this! The Privacy Browser by Soren Stoutner has this as a built-in feature, so I'm glad you're providing it as an addon for FF.
In the Privacy Browser, it not only offers the option to remove "&fbclid=" and "?fbclid=", but also "&fbadid=" and "?fbadid=". Then there is Google Analytics ("&utm_" and "?utm_"), and Twitter's AMP redirection ("?amp=1").
Maybe you can extend this your addon to one mighty ulr-cleaner… :D