I’m having this very strange issue, where a small green dot shows up in the tab bar of Firefox. Opening a new tab makes it go away, and closing Firefox also makes it go away. I tried taking a screenshot, but it didn’t show up. It only being visible when Firefox is open makes me think it isn’t a dead or stuck pixel. I’ve had the same issue on Firefox and Wayland, and through two fresh installs. Also, just got my laptop about a week ago.
If you are referring to a dot like in my screenshot, the explanation is simple. The dot shows up every time, when more than one instance of an app is running. In this case I reproduced it with a normal Firefox session and an additional instance with a private/incognito tab.
Opening two instances of dolphin for example will also add that green dot.
If I have a theme with white titlebars (e.g. Breeze Light), I can’t see it. Funnily enough, normal Breeze (with grey titlebars) was making it show up in system settings, but Dracula doesn’t. This is just bizarre.