Hit the Alt key or the F10 key to see the Menu bar temporarily.

Or right-click to the right of Tab, and then click on Menu Bar to have the Menu Bar always showing.

In current Firefox releases (29 and later), that have have the Australis user interface, the orange Firefox menu button has been replaced by the three-bar Firefox Menu button at the far right end of the Navigation Toolbar and this button is always visible, whether you have the menu bar visible or hidden
A consequence of this location is that you no longer can hide the Navigation Toolbar

• It is still possible to have the menu bar visible via the right-click context menu on the toolbars to have menus like the File menu with Print (Ctrl+P) and Print Preview and the Bookmarks menu
• There is a star like button next to 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.
  You can find "Show All Bookmarks" to open the Bookmarks Manager (Library) at the bottom of this drop-down list
• If you bookmark a page then "Bookmark This Page" in the Bookmarks menu changes to "Edit This Bookmark"
• You can make the the title bar visible via the "Title Bar" button at the bottom left in the Customize palette window

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

You also can use Ctrl+p to call up the print dialog when the menu bar is not visible. Handy for pop-up windows, and works in many Windows applications.