What is this icon and how can I get rid of it?

Hi, sorry about that: it's at the top left corner of the firefox browser, covering the buttons used to close the window/minimize/maximize. It eventually went away when I force-quit the browser and reopened it, but it had previously gone away and come back, so I'd like to disable it completely. It's only since the most recent update. When it's there I can still close/minimize/maximize using the toolbar menu, but why on earth would I want to do those extra steps every time?

To conclude: what is this and how can I get rid of it?