I have a colleague whose Mac has the same issue as you (2015 MBP 15" Retina). Chrome runs awesome for him, Firefox crawls. However, I have the same model and for me Firefox is as snappy as anything.
You're right that this needs to be fixed, but its lame to assume that Firefox devs do not test on Macs. You're simply experiencing a bug that is hard to reproduce, that is all.
Firefox is snappy at the beginning, but starts getting really slow and clunky after about 8 hours, or sometimes 2-3 days of the laptop being suspended and then turned on again. I can make it fast again if I close the browser and re-open it. But it's not slow right out of the gate.
Also slow is relative. If someone hasn't had an experience with a faster browser, then "slow" just feels normal. I had that after using Firefox for ~9 months. It just felt normal, and then got a bit slow and unusable after a while. After switching back to Chrome on Mac, it was like a breath of fresh air. I forgot what a responsive UI felt like.
I have sympathy, debugging is hard! But it's weird that across hundreds of threads no one from the dev group has anything to say, or that there is not disclaimer or "ask for help" or whatever.
"Yes we are aware that a certain percentage has severe performance problems on osx, we believe this has to do with the Retina displays blah blah" - instead it seems like no one from the dev group has been following this problem when you ask them. Despite weekly threads, just like the posts here on Hackernews in every FF thread.
Possibly because complaining on hacker news and calling people incompetent on threads they have no reason to personally follow isn't an optimal strategy for receiving tech support.
I will admit it was too harshly written, the issues seems to only be on the osx platform and Firefox is great on all other platforms, but the issue has been detailed in their own tracker and subreddit for many years.
