OP here! Dang it! What OS and version of Firefox are you on, if you don't mind my asking?
I hear ya about the scroll, and that's valuable feedback. Are you using an actual mouse or a trackpad?
Latest Firefox on Manjaro - pretty sure it's to do with with the graphics driver and Firefox not playing nicely (fine on Chrome) - so I wouldn't worry too much! (There's some webgl config settings I think...)
Using a mouse, so the scrollwheel to move it seemed ... off ... but then changing the blur intensity made it do what I would expect (letting the mouse move the image) so that's all good! :D
