I did the same thing. I gave FF about a week before I got frustrated and went back to Chrome. FF is still noticeably slower in terms of basic UI actions (opening/closing tabs, menus, etc.) and every hour or so it would start hammering my CPUs. I don't know why.
Have you looked at about:performance and also when you hit this, then create a perf profile using https://perf-html.io/ and submit it as a bug to bugzilla.mozilla.org. If possible do both using Nightly. In my experience the developers are very(!) welcome to good bug reports.
