Steer Mouse and the Middle Click button
I was shopping online for a replacement for my bluetooth wireless mouse for mac, and I read that there was an alternative mouse driver that might let me tweak the acceleration settings to something more acceptable.
I ended up downloading and trying Steer Mouse, and it’s actually pretty good at doing that. You can bring down the “Tracking” and bump up the “Sensitivity”, and it won’t be as wonky. Qualitatively, there’s also less noticeable lag between mouse movement and actual input (something that’s been driving me crazy as well). It may hold up better in heavy application use as well, time will tell.
However, one thing I wasn’t willing to live with was the default remapping of middle click to “Move to Close Button”. To change it to work with Firefox new tabs again, you can remap it to the Click action with a apple-key modifier. Seems like that works again. I’ll let it go through the trial period and see if I still like it after some more use.

Dont bind it to (Click+Command) but just map it to imitate “Middle Click” in SteerMouse.