Solved the issue adding rules to /etc/apparmor.d/usr.bin.firefox:

# Native Messaging
owner @{HOME}/.mozilla/**/native-messaging-hosts/** ixr,
/usr/bin/keepassxc-proxy ixr,
/run/user/1000/org.keepassxc.KeePassXC.BrowserServer ixr,
/run/user/1000/app/org.keepassxc.KeePassXC/org.keepassxc.KeePassXC.BrowserServer wr,

and sudo aa-enforce /etc/apparmor.d/usr.bin/firefox

Thanks


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Not connect for Firefox (not Snap) on KDE neon

Firefox 111.0.1 (64 bits) Jammy org.kde.neon.net.launchpad.ppa.mozillateam
KeepassXC 2.7.4
KeepassXC-Browser 1.7.11