Was a little bit difficult to understand how to use it for Mozilla account:
Find it in https://support.mozilla.org/en-US/questions/1276862:
Create a Keepass entry for Firefox and highlight it in the list
From the menu bar select [Entries->TOTP->Set up TOTP...]
Display the QR code in the FF set up web page and copy to the [Secret Key:] field from above.
Accept defaults and press [OK]
From the menu bar select [Entries->TOTP->Show TOTP..].
copy this number into the response field in the FF set up web page

It should now be all set up.
To use highlight the entry and select [Entries->TOTP->Show TOTP... ](or copy TOTP... if easier)

Excellent module

Overall, the browser integration for KeePassXC works well. Sometimes there are some issues with autofill when the site has you enter a username, then a password on the next page, but overall it works pretty well.

Works only for a part of sites. For all others just can't fill in credentials. Also when creating a new account a pop-up bar displayed to store new data. But click in it produces error "can't save credentials". Meanwhile the database is connected and unlocked.
So I decide the plugin to be quite buggy.

works perfectly !

Unintuitive to complete uselessness

**SOLVED**
While the main client appears to work fine, the browser extension is useless on my system - nothing happens when clicking the icon in a username field, and when using the right-click menu to fill username and password, again, nothing happens. The database is unlocked and detected as connected, and browser integration is enabled, so I'm at a loss as to what to do to fix it.

edit: autotype works, but this is unrelated to the extension.

edit2: Somehow, it's all working now, one particular site was fixed by removing its subdomain in its entry, but I've no idea why all the others started working - one of the restarts or extension reinstalls must have done the trick

Works really well on Firefox across both Windows and Linux (Fedora 33). Only thing I don't like is that sometimes I can't get the banner to go away or save a new account and password, but I think it's something that's borked with my PC's configs.