I agree. Here's my setup for standalone/isolated profiles if it's any help:
"firefox.exe" -no-remote -fullscreen -P "spotify-profile" -url "https://open.spotify.com"
and user_pref("taskbar.grouping.useprofile", true) so that I get separate icons in my taskbar for each profilehttps://developer.mozilla.org/en-US/docs/Mozilla/Command_Lin...