Can I show keyboard shortcuts (hotkeys) on Mac OS X?
You can see system keyboard shortcuts in System Preferences > Keyboard > Shortcuts (or by searching Google).

For keyboard shortcuts that correspond to menu items, they can be seen in the menus at the top of the screen, across from their corresponding commands.

Regarding the contextual menu, I just tested with the Desktop, Google Chrome and Apple's Messages. Pressing a letter takes me to the first item that starts with that letter. (For items that start with the same letter, also type the second [and third, etc.] letter to jump to the correct item.)

I don't believe Firefox's behavior is normal on Mac, so I would say filing a bug report with Firefox would be the most appropriate course of action. I'd think changing to how the rest of the OS works would be ideal. Short of that, putting the shortcut right-aligned across from the corresponding item would work best, since that would match how menu bar menus work.