I’m pretty sad that it took a long time to solve this. I know there was a lot of work put into bringing Firefox back up to snuff, but this single issue seems to have caused a lot of trouble. This Bugzilla link is light grey for me, so I’ve visited it before, most likely a year or two ago. Since then I’ve been running Firefox with the gfx.compositor.glcontext.opaque flag set to false, which doesn’t even cause any issues aside from sharp corners on browser windows, and HTTP auth prompts having a black background instead of a nice macOS blur. That was the best workaround available, one I almost wish they could have made some compromise to enable by default in the meantime. I’m glad it’s been sorted now, but this seemed like it should have been a release blocker to me when I first started switching back to Firefox during the Quantum betas.
It only really affects Retina Macs, which are a small fraction of the total userbase [1]. I understand that it's frustrating, but it would have been doing a disservice to the vast majority of Windows users to block a release on Mac power improvements. Especially since graphics power management on macOS isn't a new issue by any means; Quantum didn't introduce it.