If your problem is related to the fact Firefox has been installed through snap, hence not "seeing" the companion app, I can insure those 2 solutions work. Another solution suggested by a user on github: run Firefox from its top level file path: /snap/firefox/current/usr/lib/firefox/firefox (sandboxing benefit of using snap is also lost).
Another common issue with installing the companion app on Linux is if you first install the app at user level then change your mind, remove the coapp files and reinstall it at system level. If you did not take special care, the user manifest files (describing the app to the browser) are still pointing to the now missing coapp binaries and those manifest files get precedence over the system ones. Solution: search for files named net.downloadhelper.coapp.json in your home directory and remove them.