I'm on Ubuntu on a Thinkpad that has the trackpad disabled. Plain Jane Microsoft wired USB mouse plugged in.

I use the right-click/context menu a fair amount. Since FF screenshots was added, it gets triggered frequently when I'm popping the right/click menu on a page -- without my making any click after the right-click to open the context menu. And with the trackpad disabled and using a mouse with a physical button and a rather distinct click, I can say this with considerable confidence.

So, then I have to to hit the escape key to kill the FF screenshot functionality. Which is becoming muscle memory, at this point.