Welcome to Australis, Mozilla's attempt to streamline Firefox and bring a common look to PCs, tablets, and mobile phones. Mozilla is trying to modernize the browser, speed it up, and make it easier to use, but still keep the browser's customizability.
Australis, Mozilla's overhaul of Firefox's user interface, gives the foreground tab new emphasis and makes the others fade more into the background. To the left of the tabs, pinned tabs that users might want to access often show just by their narrow favicon images. The entire menu has been reduced to a customizable toolbar with a few buttons on the upper right of the interface; clicking the three parallel horizontal lines opens up the menu.