When Australis came out, it looks like they forgot to give the bookmarks bar the signature Australis look. This addon seeks to do just that. It removes the label from the bookmark item, creating much more room but it doesn’t clutter the toolbar. It avoids this clutter by giving the bookmark buttons the same styling seen on the “Navigation Bar” buttons.
Here is a little more about this add-on from the developers:
Because there is no label and the tooltip was disabled (tooltips are not very Australisy), the panel at bottom left (sometimes bottom right) of screen that shows the link that you are hovering over, is made to include the title of the bookmark.
Want to learn more? You can pick up the AustralisBookBar add-on from the Firefox Add-ons website.