AutoScroll in Firefox broken by BTT

In the process of setting up a new mac mini m4, I installed 2 browsers: firefox and brave because they both support vertical tabs and AutoScroll (which is when you hold down the middle button and scroll around the page-- I use an external mouse). AutoScroll worked perfectly on both until I installed BetterTouchTool and tried to setup 2 shortcuts for all apps: Cmd + Scroll Up -> Cmd + =, and Cmd + Scroll Down -> Cmd + -; which is to say holding command and scrolling with the mouse wheel increases / decreases the font size for many apps, particularly browsers.

After this change, AutoScroll in Brave still works fine, but AutoScroll in Firefox is messed up-- it usually just flashes the scroll icon when I hold down the middle button for a fraction of a second and then goes off (without me releasing the middle button), but sometimes the scroll icon will stay for a second or two and AutoScrolling does work in that brief time. I removed these 2 mappings, but it had no effect, then I removed BTT and restarted, but it also had no effect.

ChatGPT thinks BTT was responsible somehow, as do I (although i cannot prove it 100%):
→ This explains why Brave (which uses Chromium) still works: Chromium handles mouse input directly using raw input streams.
→ Firefox respects macOS' higher-level event streams more closely and is now receiving inconsistent signals for middle button "hold".

Is there a way I can be sure I have removed all vestiges of BTT so that nothing remains-- either any of its components, or any of the settings it may have changed during the install or mapping configuration?

once you quit BTT, it doesn’t do anything anymore. If it still doesn’t work while BTT is quit, it is not caused by BTT

In fact, it was not BTT: using a bluetooth mouse Firefox had no problems with Autoscroll, and when I flipped to another (windows) machine on the same KVM switch, Firefox had exactly the same problem with the trackball that had the problem, and Edge Autoscroll worked fine.

I swear, I am going through cheap Chinese wired trackballs at a rate of about 3 / year. When Logitech made them I could get 3 years out of one.