Allows you to move the Firefox bookmarks bar so it shares a line with other toolbars or tabs while staying right-aligned and without creating excessive amounts of empty space.
Here are more details about the Right-Aligned Bookmarks Bar add-on for Firefox:
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 center 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.
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. Pick it up at the Firefox Add-ons website.