The Firefox bookmarks toolbar is designed to take up a whole line but if you only have a handful of items on it, and only have icons showing and not names, this is excessive.
Normally, if you try to move the bookmarks toolbar to another line you will find it wastes lots of empty space, reduces the size of other elements (e.g. tabs or the address bar) on the same line, and wants to be left or centre aligned (depending on what is to the left of it). Another annoyance is if you right-click the empty space it adds you get a menu about bookmarks instead of the usual toolbars/customize menu.
This very simple extension changes a single attribute on the bookmarks toolbar to stop all of that happening. It will now only be as wide as the items within it.
There are no settings or UI; just disable or remove the extension if you want to revert to normal.
A while after writing this, I found there is another similar extension written before mine. I haven't tried the other one, so I don't know if it's exactly the same, but here it is in case you prefer it (I don't care which one you use :)):