Comment 58 for bug 237941

(In reply to Dave Martin from comment #50)
> I've highlighted this bug many times and (have just checked) my first report was 18 years ago !!!! in bug 118394 which is still open. Eighteen years, please, let's have a fix !! This bug and the lack of a vertical tabs option in base Firefox (I use a 3rd party addon, tree style tab, wonderful) are major detractions to this browser.

Just let it go, not going to happen. Use the above mentioned for this functionality, and other useful ones such as jumping to the searched bookmark/folder's location on the tree: https://addons.mozilla.org/firefox/addon/bookmark-search-plus-2/
It opens in the sidebar the same way as the built in sidebar.