It’s been 2 weeks now my 1Password extension is not working on Firefox. The button is not clickable even after upgrading to the latest Firefox version. After asking help from 1Password on Twitter, it turns out that the fix is as easy as clicking a menu item.
In 1Password main app’s menu, click on Help -> Advanced and select Use Native Messaging Protocol. 1Password button on Firefox should work at this point. Maybe restart Firefox just to be sure.