Firefox 29 and later versions have the Australis interface that looks different than the user interface in Firefox 28 and older versions.

• With the Australis interface the orange Firefox menu button has been replaced by the three bar Firefox Menu button at the far right end of the Navigation Toolbar
  A consequence of this new menu button is that you no longer can hide the Navigation Toolbar
• There is a star like button to the right end of the search bar on the Navigation Toolbar to bookmark the current web page and a "Show your bookmarks" button next to it to open the Bookmarks in a drop down menu
• It is still possible to have the menu bar visible via the right-click context menu on the toolbars to have menus like the Bookmarks menu

See also:

• https://support.mozilla.org/kb/how-to-make-new-firefox-look-like-old-firefox

• https://support.mozilla.org/kb/common-questions-after-updating-to-new-firefox
• https://support.mozilla.org/kb/learn-more-about-the-design-of-new-firefox

• http://forums.mozillazine.org/viewtopic.php?f=38&t=2824649

Even Internet Explorer gives you the chance to remove an update if you don't like the effects it has. Is this possible with Firefox? And is there a way to have Firefox ask before automatically installing such updates?

Yes, you can go back if you wish. https://support.mozilla.org/en-US/kb/install-older-version-of-firefox Yes, when updates to Firefox are found, you can set it to ask you what want to do. https://support.mozilla.org/en-US/kb/advanced-settings-browsing-network-updates-encryption#w_update-tab