**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.

I love it. Much more friendly!

Thanks for the welcome. I took the dummies way out and just removed it. Thanks for the help.

As a KeePassXC user, this extension is absolute gold. Getting it to work isn't entirely intuitive. Tips from a happy user:

Make sure the KeePassXC database has permission to link by checking off the relevant boxes under Tools > Settings > Browser Integration and Database Settings > Browser Integration. Don't close KeePassXC. Go to the extension and connect it under Settings > Connected Databases. In order for this extension to function, KeePassXC *must be running*. In KeePassXC, you can choose to minimize or hide the program instead of quitting under Tools > Settings. In the KeePass entry itself, don't forget to specify a URL.

Like it! brings all my passwords together in one place, where I can control them.
TNX Guys for the extension.

No matter how many times I click on the add database button it will not work, rendering the Add-On useless

It's really a convenience to use this add-on with KeePassXC. But, there been an issue that has been raised several months back :

In my case
Firefox 83
KeePassXC-Browser 1.7.3
Ubuntu 20.04 LTS

`keepassxc-proxy` makes tcp connections to remote servers over internet & transfers data(with 36+ KB/s speed) without user permission under Linux root privileges.

tcp 0 0 192.168.42.245:52470 117.18.237.29:80 ESTABLISHED 20559/keepassxc-pro
tcp 0 392 192.168.42.245:34738 209.216.230.240:443 ESTABLISHED 20225/keepassxc-pro

tcp 1 0 192.168.42.245:49194 172.67.146.206:443 CLOSE_WAIT 20225/keepassxc-pro
tcp 1 0 192.168.42.245:53060 104.20.214.50:443 CLOSE_WAIT 20225/keepassxc-pro
tcp 1 0 192.168.42.245:60362 172.67.153.104:443 CLOSE_WAIT 20225/keepassxc-pro
tcp 25 0 192.168.42.245:39074 151.101.113.137:443 CLOSE_WAIT 20225/keepassxc-pro
tcp 7890 0 192.168.42.245:34738 209.216.230.240:443 CLOSE_WAIT 20225/keepassxc-pro

simple ip2location domain :
104.20.214.50 .... CloudFlare Inc.
117.18.237.29 .... edgecast.com
151.101.113.137 .. fastly.com
172.67.146.206 ... CloudFlare Inc.
172.67.153.104 ... CloudFlare Inc.
209.216.230.240 .. M5 Computer Security

I tried several times, had to lock password-DB out of fear.

This bug/issue is not addressed by Firefox, claim KeePassXC team on their issue-tracker-logs.

I'm in a limbo, is Firefox breaching user privacy without their knowledge; what data is being transferred to remote servers by keepassxc-proxy?
Should I abandon Firefox; isn't it the most secure (or configurable) browser out there?
Should I change passwords of hundreds of accounts now?

I'm using AutoTyping where ever it's possible & often wished they had auto-type separately for Username & Password; you know for pages like firefox or google account logins.

cheers