' | sudo tee /etc/apt/preferences.d/mozilla-firefox We need to ensure that packages from Mozilla's PPA have higher priority (otherwise apt might 'upgrade' to the Snap again).

Ubuntu provides a transitional firefox Debian package which installs the Snap.

This isn't pretty but the following approach worked for me:

The KeePassXC Troubleshooting guide recommends installing the browser from the official PPA. While work on a solution is in progress, all we can do is work around this issue (or switch to a distro which still provides browsers as regular packages like Debian). Unfortunately this issue can't be fixed on KeePassXC's end and so far neither Flatpak nor Snap support NativeMessaging, yet (their sandboxing techniques prevent the browser extension from talking to the KeePassXC process).

There is also a German article by Heise Online with a step-by-step guide on how to get this going - in case you want to give it a try. Beware that this is pretty fresh - the code hasn't been merged upstream, yet, so don't expect this to be production-ready. Update: Native messaging support in the Firefox snap is now available as beta.

Keepassxc browser plugin