AFAIK The functionality you're talking about is hard coded into the Firefox core and not a plugin. I think you meant Add-on but either way.
Ctrl+Enter for .com
Shift+Enter for .net
Ctrl+Shift+Enter for .org
Adding Alt to any of those shortcuts will open it in a new Tab.
These are all built in and can't be modified without downloading the Firefox source, modifying the one value you want, and then recompiling it.
Get the source (listed as Mozilla Source in the menu on the right)
Simple build Firefox
I would use a
one liner to search the source for "http://www." since that is part of all of the shortcuts.
Not sure if you want to get that in depth.