See here for some more info. Enter you bibsonomy username and click install - then find it in your bookmarks folder.
First time you use it you will have to allow it to do cross-site XMLHttpRequests.
For obvious reasons this will only work in firefox/mozilla.
Oh, and sometimes it will break a bit if you type too many things quickly... wait a few seconds and try again :) - i can't be bothered to fix it now, I've wasted enough time today.