I find it mildly amusing that there's a profiler in the devtools. And yet there is another profiler that is at times more powerful than it. I have used the gecko profiler from time to time to identify which feature the page was using that slowed down the entire browser. Multi-process means just debugging a single tab doesn't give you the whole picture.
To any other devs who like me who want to use Firefox Debugger, but were driven back to the devil Chromium by need for consistent, seamless copy-paste and select-middlemouse interaction: Give 70.0.1 a try. Much, much better. So much that I've dropped Chromium. (Ubuntu 18.04)