Menu differences in Windows, Linux, and Mac
From MozillaZine Knowledge Base
Most (if not all) of the knowledge base articles related to Firefox, Thunderbird and SeaMonkey refer to the menu sequences used in Windows. In the Linux and Mac versions of these applications, however, the menu sequences sometimes differ, as shown below.
Firefox and Thunderbird
Note: Starting in Firefox 4, the Menu bar is hidden by default on Windows Vista and above. To access Options (and other menu items), either click the [≡] button in the right corner of the main toolbar (for Firefox versions previous to 29.0, click the Firefox button in the upper-left corner of the title bar) or press the "Alt" key to temporarily show a hidden Menu bar (check View → Toolbars → Menu Bar to keep the Menu bar visible at all times).
Recent versions have a summarizing menu to access frequently used items. This is either an application button located in the upper-left corner of the title bar or, starting in Firefox 29.0 and Thunderbird 17.0, a [≡] button in the right corner of the main toolbar.
If you come across other menu differences, feel free to add them to the above.