I'm sure there are a lot of Firefox extensions of questionable quality but I would expect that at least the popular ones are of fairly decent quality. I also would assume that only a small minority of users know about and actually use extensions.
Even people who do use extensions probably don't really use more than an adblocker.
An extension doesn't have to be popular to be significant. 1% of people accidentally installing some spyware crap that slows down the browser by 50,000% is going to move the needle.
Even people who do use extensions probably don't really use more than an adblocker.