Gmail notifier add on

I have done my best to find where to place this question.. my system is desktop High Sierra 10.13.6 firefox 115.7.0 iMac 21.5"

As gmail notifier is no longer available I've tried adding an extension, it is called a "Restartless " version

It is Enabled, it has permission to connect with google and I have checked Preferences

I have pinned it to the toolbar

However the badge/icon is black instead of red and isn't working

Systems Preferences says there aren't any extension installed which could be the problem but there weren't any instructions to install it.

I've restarted the computer just incase it would help but it hasn't

I would very much like to have this, or similar extension as I miss the notifier

It has good reviews

I operate my own computer well but am not technical with inner workings therefore would appreciate a simple solution, if there isn't one please let me know and I will remove the extension.

Regards, Susan