Add elements to browser interface

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.

Currently, extensions are limited to one toolbar button (either on the main part of the toolbar, or in the Page Actions area of the address bar) and can’t create their own bar. If I remember correctly, the question of whether extensions should be allowed to get more real estate in the UI (Toolbars API) was put over to next year; it’s not being considered for implementation in 2019.

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Mmmm well, the truth would be good to be able to do this, would expand the possibilities at the time of developing, thanks for the information, solved my doubt, you can close the subject.

The provided list was of course not everything, add-ons can e.g. also use sidebars for more screen space.

See this page for all possibilities:

Some add-ons also emulate tool- or message bars by injecting code into the current website, to display a bar created with HTML/CSS/JS at the top or bottom of the website, so it looks as if it is a browser bar. This add-on e.g. does it AFAIK.