This is your profile icon. You can go to your profile manager and change it.

Hello Ty, thanks for your quick answer. Is there a way to remove the icon, not changing it? It wasn't there since yesterday, I think. And I don't like that feature, so I want to remove it.

If you have more than one profile, an icon is obligatory. But I guess you can add an empty image there.

That's it :-) I must have clicked "something" when Firefox started yesterday and by that created a 2nd profile. There was a kind of campaign popup coming up, probably advertising profiles. Not nice, but at least I found the reason now. After I removed that 2nd profile, the icon was gone.

Thanks a lot!