How to setup Firefox and KeePassXC in a flatpak and get the KeePassXC-Browser add-on to work?

If KeePassXC is sandboxed [in a Flatpak](https://flathub.org/apps/details/org.keepassxc.KeePassXC) , browsers can only access it, if they are _not_ sandboxed, i.e. installed as an deb/rpm package or similar on the host. Sandboxing both the browser, i.e. [Firefox](https://flathub.org/apps/details/org.mozilla.firefox) , and KeePassXC – or at least the browser and installing KeePassXC natively, which you'd actually want for security reasons – is not possible. TL;DR: * this should work out-of-the box: Firefox (host-installed), KeePassXC (flatpak from flathub) * this does not: Firefox (sandboxed), KeePassXC (host or sandboxed, does not matter) So how to make that communication work?