1. Record a profile with the Gecko Profiler WebExtension (Install it from https://perf-html.io/)
2. Upload the profile to https://perf-html.io/
3. Create a bug on https://bugzilla.mozilla.org/ with a link to this performance profile.
The Firefox developers are friendly and welcome, and I'd say in about 50-75% of cases the bugs get fixed in a timely manner.
It might also be that being the second fastest guy on the track will not win you a gold medal. As everyone tends to develop and test software on Blink/V8, it will undoubtedly feel slow when ran on a competiting browser.
If it being slow really is a rule rather than an exception, then perhaps there is something in your configuration making it that way. Or a specific website that you happen to always have open. It's not normally always slow for most people, so again, this would be useful to know about.
It's true that often developers only test on Chrome, and in making sure it runs smoothly there they happen to make something that performs badly on Firefox. If we know about these websites then we can see if it's something we should change on our end, or get in touch with the developers ourselves.
How about making this, Record, Profile, and Submission procedure built in. Button Accessible within 3-4 clicks. The amount of work required to get all these done and submitted is not easy especially for average users. I do a lot in my most enthusiastic days ( Mostly for Memshrink ), but after Memshrink I couldn't be bother much, unless the bug is annoying enough that persist.
If the bugreport just reads "it's slow", then of course that's not helpful. However, performance issues as drastic as described above are certainly not normal. I'm using Firefox on Windows, Linux and (up until recently) macOS and never encountered anything remotely like this. So it would certainly be helpful to get detailed traces of that abnormal situation.