Probably this one [0], though I dislike the branding at the top of the sidebar and the fact that since FF lacks native support, you'll still have tabs on top of the window.

[0]: https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...

You can disable those tabs on the window, you just have to mess with the browser chrome's CSS yourself. Which is relatively easy, but basically the reason I haven't used it so far.

But now that you mention it - I'm going to give it a shot now.

Here's a list of tweaks that you can try: https://github.com/piroor/treestyletab/wiki/Code-snippets-fo...

Thanks! I was looking for the best snippet to hide it, and this one is a bit more specific than the one I was using. (Plus, it allowed me to hide the bar in full-screen.)

It used not to be like that but the last versions of Firefox have messed up pretty hard with it.