Firegestures had 270k users, according to AMO. A quarter of a MILLION people.
You broke mouse gestures entirely on MacOS and Linux, and didn't allow them to work well on Windows (DOM needs to load before gestures can be used because you force script injection, don't work on internal pages, don't work on top of browser chrome, etc).
Playing the devil's advocate: the top two most popular extensions (as listed on [1]) are Adblock Plus at ~14 million, and uBlock Origin at ~4.2 million users. That means Firegestures has about 2% of the top extension, and about 1.5% if you combine the two (assuming not many people use both ad blockers at the same time, especially since they use the same lists). That's just the people with those extensions. And it appears that that's active daily users (as opposed to people have downloaded it at some point in a Firefox that is no longer being used).
I do agree that Mozilla has handled the transition terribly; they should have made the API available first before removing everything. That way they would at least have the excuse of it being the add-on authors not cooperating. The way they've done it, before actually making the things possible, just makes it look like they're arrogant.
Firegestures had 270k users, according to AMO. A quarter of a MILLION people.
You broke mouse gestures entirely on MacOS and Linux, and didn't allow them to work well on Windows (DOM needs to load before gestures can be used because you force script injection, don't work on internal pages, don't work on top of browser chrome, etc).