Nach der Installation von Firefox (snap) und KeePassXC (snap):

apt install xdg-desktop-portal

Erstellen der Verzeichnisse und der Datei ~/.mozilla/native-messaging-hosts/org.keepassxc.keepassxc_browser.json mit folgendem Inhalt:

{
    "name": "org.keepassxc.keepassxc_browser",
    "description": "KeePassXC integration with native messaging support",
    "path": "/snap/bin/keepassxc.proxy",
    "type": "stdio",
    "allowed_extensions": [
        "keepassxc-browser©keepassxc.org"
    ]
}

Danach Firefox neu starten.