Well, I didn’t really know how to edit the title to express well what I want, so I’ll gladly receive some comments. Well, I say the following, in very old versions of Firefox (when XUL was used) it was possible to add elements to the browser interface, I attach a photo to explain me better.
The screenshot is from the Pale Moon Navigator, based on the Gecko engine if I’m not mistaken. Then, as you can see in some way was added an extra bar to which they were adding elements and was left that way, not exactly what the tool in question works but if you press the blue button in the top right corner the bar disappears and appears, my question is: Is it possible to do this in current versions of Firefox when developing extensions?.
My question arises because all the extensions I have used are based on Popups that are deployed by pressing the button of that extension. Then I think we could do some interesting projects if this possibility existed at the moment.