My previous opinion was wrong. I realized that sometimes it collides with browser's native password management. And of course the extension is very sensitive to url format, including correct protocol (https vs http) which is actually a virtue! After a bit of struggle I must say that this extension performs very well (on Debian)

I'm leaving my old opinion, maybe it will help someone:
Does not fill username nor password on almost all sites (keepass icons on user/pass fields is sometimes grayed out, sometimes not but it doesn't matter. Firefox or Chrome? Doesn't matter. Fill option doesn't work, keyboard shortcut doesn't work when database is connected (but password generator works!). No KeepassXC option in right-click menu. Surprisingly it works correctly for github (the only working site I found in my basket) which ensures me that I perfectly know what I'm doing.

It is worked with Ubuntu 22.04.1 LTS now.
I just had to execute command:

flatpak permission-set webextensions org.keepassxc.keepassxc_browser snap.firefox yes

Great addon!