I had the same problem in Firefox for MSW10 (uBlock Origin did not help) and somehow my Chess.com notification settings were reset to default.
I resolved the issue by setting History to "Always use Private Browsing." If you do this, be sure to configure your add-ons to work in Private Mode.
Funny, I also use Firefox, and it worked for me. (I'm using Linux, but I wouldn't think the OS would matter.) uBlock Origin won't automatically block the notification popup - You have to do a one-time manual block of the element.
I had the same problem in Firefox 73.0.1 for MSW10 (uBlock Origin did not help).
I resolved the issue by setting History to "Always use Private Browsing." If you do this, be sure to configure your add-ons to work in Private Mode.