Cooliris kind of does that thing:
http://www.cooliris.com https://addons.mozilla.org/en-US/firefox/addon/5579
Not sure that's quite what you mean though...
I like the 'Answers' plugin:
https://addons.mozilla.org/en-US/firefox/addon/735
Alt-click on a word and it gives you a pop up with information on it. Very handy :-)
Greg