To pause the video you would press the play button. The back/return button pressed one or more times will land you on the home window. You can tweak it to your heart’s content though.
The following should provide you with sufficient information.
It is not quite as bad as it may seem. Basically you figure out the name of the window that you want to change mappings for, fullscreenvideo in your case, then navigate down to the kind of action you want to use, and then once you find the action you want you click on it and when prompted press the key you want assigned. However, the home button might now work with that add-on as I think it takes you out of the popup instead of recording the key press. If this is the case the modification would need to be done manually.