Hey Dave,
That action is for working with menus in the menu bar.
Keyboard Maestro has no direct ability to select items in pop-up menus.
In many apps you can use type-select with an Insert Text by Typing action to get where you want, but Firefox (and some other apps) does NOT respond normally to type-ahead in the contextual menu.
JM's scripted method WON'T work for you, because Firefox uses unconventional coding methods that AppleScript UI-Scripting cannot see.
You're only real choice here is to use the Click at Found Image action to select your first level contextual menu item and then go on from there.
By the way – you really need to tell us what app you're working in, so we don't have to guess.
How Do I Get The Best Answer in the Shortest Time?
-Chris