The Classic Theme Restorer will restore squared tabs, appmenu, add-ons bar, small button view and more on Australis UI (Windows/MacOSX/Linux + Fx 29+).
Here are more details about the add-on in question:
This add-on restores squared tabs, appbutton/appmenu (known from Firefox for Windows), bookmarks menu button, bookmarks sidebar button, history sidebar button and other buttons from Firefox 4-28 (non-Australis) on Firefox 29+ (Austalis UI – Windows, MacOSX, Linux). All buttons are fully optional. Some of them have to be moved to toolbars manually using browsers ‘customize’ area.
Want to learn more about this add-on? Check out the Classic Theme Restorer at the Firefox Add-ons website.