Only trouble is I had to go to relatively great lengths to hide my horizontal tabs: enable userChrome stylesheets, go to about:support to find my Profile folder, create a chrome folder, create a userChrome.css file and paste contents from [1], and restart. So now I have only the side bar of tree tabs, and a hot key to hide/show. Screencap attached [2] (using classic theme for windows from here [3])
[0] https://arc.net/
[1] https://github.com/piroor/treestyletab/wiki/Code-snippets-fo...
[2] https://coltenj.com/wp-content/uploads/2023/07/Screenshot-20...
[3] https://github.com/malvinas2/ClassicThemeForWindows10/tree/m...