It could be easier, but still doable in 5 minutes (that's how long it took me) with some CSS, see https://github.com/piroor/treestyletab/wiki/Code-snippets-fo... on how to do it.
There seem to be some security concern or it's not so easy UX wise, see https://bugzilla.mozilla.org/show_bug.cgi?id=1332447 for all the discussions and progress.
