Keyboard Savior Xtreme
Keyboard Savior Xtreme is a Firefox add-on that blocks websites from hijacking your keystrokes. It takes the extreme approach of blocking sites from seeing any keyboard event except when focus is in certain editable areas.
Too extreme for you? Check out the related tools.
Whitelist
If you want to allow custom shortcuts on a particular site, either:
Add its domain to the list in add-on preferences
Or click the Savior’s toolbar icon:

Click the icon again to remove a site from the whitelist.