Is there a firefox addon that would display all shortcuts on a web page and also browser shortcuts.

for eg: c is the keyboard shortcut for gmail compose. So either on a particular key combination or by hovering over the compose button it should show a "c" to indicate there is a shortcut for this operation.

I guess If we keep on seeing the shortcuts popping each time when we do an action then we can start using and remembering more and more of it.

Thanks.

P.S: If this is not the forum to ask this question please suggest appropriately in comments.