I can confirm that Tempermonkey extension is available for Firefox (and Librewolf) browser as well, and Bitcointalk UserNoted works just fine with that.
Greasemonkey was not updated for years and I am not sure if anyone works on it anymore, while Tampermonkey was updated few months ago, but I think it's closed source.
I have only one request if possible, can you somehow can enable easy option to export and import notes with member usernames.