The bookmarks toolbar is known as "Places". In earlier versions of Firefox we didn't see icons and now we do. If you need more room then this addon is for you. A restartless addon that hides the images.
Next I plan on making the opposite of this addon called "IconifyPlaces" which will remove the labels from the bookmark buttons and only show the icons.
Thanks to Scot from Neocodex for the request/idea
Thanks to The Oxygen Team for the icon