I’m pretty sure ⌘W accomplishes that in those browsers. So getting End to “press” ⌘W should be all you need to do.
You could probably create an AppleScript that does that, but it might be easier to use an application that deals specifically with keystrokes, like Karabiner. You might even be able to pull it off only using the built-in key bindings system for macOS.
Also, now that I think about it, I’m not sure Quicksilver would let you assign a trigger to the End key. (It works with function keys, so maybe.)
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Rob McBroom
http://www.skurfer.com/