Good point, should have thought of that. I have a bunch of things open where I don't want to reload the page so restarting in safe mode is a bit tricky now, but unless someone happens to know the exact answer I can try this on the weekend and perhaps narrow it down, thanks :)

Some issues are caused by the current profile, so it is a good idea to use a temporary profile for testing weird bugs. Here is a script to automatically create and clean up a temporary Firefox profile:

  $ head -n-0 ~/bin/firefox-tmp-profile 
  #!/bin/sh
  # Workaround for missing feature:
  # https://bugzilla.mozilla.org/show_bug.cgi?id=1604376
  set -e
  dir="$(mktemp --tmpdir --directory firefox-tmp-profile-XXXXXXXX)"
  cleanup () { rm --recursive --force "$dir"; }
  trap cleanup EXIT
  firefox -no-remote -profile "$dir" "$@" || true

about:processes is also a decent way to pinpoint the most impactful extension.

Edit: my bad, I meant to say about:performance.