Fun fact: you can assign each separate Firefox instance a different X window class with the --class flag. Then your window manager may be configured to raise windows with that class to the top.

E.g. I have something like this in my StumpWM config:

    (defcommand javascript-firefox ()
      (run-or-raise "firefox -P Javascript --class Firefox-javascript" '(:class "Firefox-javascript")))
    
    (define-key *top-map* (kbd "H-F") "javascript-firefox")

Then all I need to do is type hyper-shift-f and the Javascript Firefox is either raised to the top or if there is none, one is run. Meanwhile, hyper-f raises my normal Firefox.

Highly recommended; I hope that I will be able to continue using it in the future. Does Wayland support window classes and the hyper key?