I confirm: it doesn't work on my Firefox either (last version 100.0, 64-bit). First shortcut key chosen: "²" (superscript 2), then others tries (whitout the quotes of course). None works. Tried also with all my other addons disabled... same behavior: not working. Perhaps something changed in the API WebExtensions of Firefox...

I set the shortcut key to 'v' - it opened tabs in the foreground not the background. Linux/firefox 76