GeekLab

A lot of websites use the right mouse button for some menu function. For instance, in Google maps, the right click is used to set destination points. Google apps uses the right click to add/remove rows/columns from the spreadsheet.

When I right-clicked, my Firefox displayed it's own menu right over the Google menu. Why?

Well, back in ±2002, when I started to use Firefox, then still named "Phoenix", a lot of sites tried to block "view source" by blocking the right mouse button. I must have configured Firefox to forbid blocking, and this config hasn't changed ever since.

Simple method

On my current version of Firefox, re-enabling sites to block right-click is configured in:
Edit » Preferences » Content tab » Advanced (right to "Enable JavaScript") » Check: Disable or replace context menus

Advanced method

Another method to switch blocking of context menu's is through about:config. This is mostly for experts and extension programmers.
dom.event.contextmenu.enabled=true = allow blocking the firefox context menu (default)
dom.event.contextmenu.enabled=false = disallow blocking the firefox context menu