only for Mac OS X, and Firefox 3.6 Rated 5 out of 5 stars
This is a plugin for Mac, and it works perfectly with Firefox 3.6. Its search feature and the zoom with two fingers on the track pad are very useful.
Unfortunately it has not been ported to Firefox 4 or 5, and I wonder if there is any intention to do it.
http://www.schubert-it.com/support/ is a possible alternative, but it lacks the search capability of the present plugin... I am considering returning to Firefox 3.6 to continue to use this precious plugin.