You are in luck as the minimum requirements for Firefox on macOS since Fx 116.0 has been macOS 10.15 so you can use the current Firefox 131.0 Release from https://www.mozilla.org/firefox/all/desktop-release/osx/ as just pick your language and download and install the dmg.

Well thank you so much James.

FYI in my experience this isn't true. There's been a bad bug on Catalina for several releases now, where many of the menus don't work (settings, bookmarks, translation, manage extensions, update reminders). I assume it's all the same component.