Pale Moon arguably isn't Firefox anymore.
If I can use the same profile if I just swap one or two extensions, then it's close enough for government work.
Honestly speaking, if the only thing you valued from Firefox was the old UI,
It isn't. It's that it's the best browser available. It might not always be fastest, but I actually find that sites work better in it... even Google sites like Youtube. I don't know how google is failing so badly at the web, but they are. Example, Youtube in chrome. Pause a video and go away for a long time. Come back and start it up again. Instead of reconnecting to the stream and buffering and picking up like normal it chokes at the end of the buffer and actually reloads the page. In the process it fails to accurately remember where you were and restarts about the place you paused last time. WTF? What was the point of having their own browser again? Certainly it wasn't to have a platform on which their own site would work correctly, because it doesn't. So just for the purposes of tracking people? Right-o!
I do run Chrome all day every day, I use it for gmail, which is still slightly better in Chrome than in Firefox. I can spare the memory. But seriously, Chrome's only real justification for existing (sandboxing) has been shown to not actually provide meaningful security, so who gives a shit?