Many context menu items in Firefox have access keys (a.k.a. accelerators or keyboard shortcuts), which let you select a menu item by pressing the appropriate key while the menu has focus. In Windows and Linux, such menu item access keys are underlined so you know what key to press, but this is not the case for Macintosh. The access keys are still there, and they do work; it's just not obvious what the access key is for a particular menu item.
This extremely simple extension highlights access keys in context menu items that have them by surrounding each access key with parentheses, like (t)his. (I couldn't figure out a way to underline the access key.)
Only access keys in context menu items are highlighted; access keys for items in the main menubar (the menus at the top of the Mac screen) are NOT highlighted, because they don't work on the Mac anyway. Note that some context menu items (especially those belonging to many extensions) do not have defined access keys, so nothing will be highlighted. Also, sometimes two or more items in the same context menu will have the same access key, because extension developers usually don't check whether the access keys they define conflict with those of other extensions. Context Menu Keys for Mac does not create access keys—it only reveals the ones that are already there; so it can't do anything about conflicting keys.