Enabling the desired behavior likely requires quite a few steps beyond just installing the extension.
https://github.com/piroor/treestyletab/wiki/Code-snippets-fo...
Originally, Firefox had a dropdown menu that allowed the user to choose whether tabs were on the top, bottom, left, or right.
This has been an annoying trend with Firefox for some time. They take the default, expected functionality, marginalize it while saying "Users who prefer the old way can enable it in a setting / extension" and then the setting gets deprecated or the extension stops working.
See also: "Classic Theme Restorer".
All the while software that might work better as an extension is bundled with Firefox and enabled by default. e.g. "Pocket", "Hello".