Note that you only see the orange Firefox menu button when the Menu bar is hidden.

View > Toolbars (press F10 to display the menu bar if necessary) Right-click empty toolbar area Firefox menu button > Options

Make sure that you do not run Firefox in full screen mode (press F11 or Fn + F11 to toggle; Mac: Command+Shift+F).

• https://support.mozilla.org/kb/how-to-use-full-screen

If you are in full screen mode then hover the mouse to the top of the screen to make the Navigation Toolbar and Tab bar appear.
Click the Maximize button (top right corner of the Navigation Toolbar) to leave full screen mode or right-click empty space on a toolbar and choose "Exit Full Screen Mode" or press the F11 key.

You are using Firefox 29. The changes used the code name Australis. The Firefox button is now replaced with a new button on the right

• [≡]
• This is now outdated Display the Firefox button menu instead of the menu toolbar
• See the Release Notes
  http://www.mozilla.org/en-US/firefox/29.0beta/releasenotes/
  • And https://blog.mozilla.org/blog/2014/03/20/help-test-the-new-firefox-beta-faster-simplified-and-easier-to-customize/
  • You may be interested in explanatory blog
    https://blog.mozilla.org/ux/2013/11/australis-is-landing-in-firefox-nightly/
• This article is updated already Get started with Firefox - An overview of the main features
  If necessary use the selector or this link specifically to see the Firefox29 (Win7) version https://support.mozilla.org/en-US/kb/get-started-firefox-overview-main-features#firefox:win7:fx29

You could revert to the Release channel and still have the Firefox button for a few more weeks or just try the new button as a lot of effort went in to the new design. You may grow to like it. If not then addons usually are able to remove some feature regressions have a look at this one

• https://addons.mozilla.org/firefox/addon/classicthemerestorer/

If you wish to make a short comment on the changes please use

• https://input.mozilla.org/feedback
  • comments being collected in http://input.mozilla.org/

Oh yay, another terrible change veiled in the name of progress no one wants, just like all the search engines I had been unable to remove from firefox mobile for months before finally uninstalling. Looks like it's time to just move on to something else completely.

hello alfador, in the meanwhile it is possible to remove search engines again. since your reply isn't really related to the already solved question though, i'm going to lock this thread...