Rated 2 out of 5 stars
This add-on apparently causes problems wih Panorama (the tabs get mixed up) which is fixed by uninstalling/disabling it. See bug https://bugzilla.mozilla.org/show_bug.cgi?id=646390 (I reproduced it on my install). Please fix =)
Otherwise, it works great (except, as mentioned by others, that it doesn't pick up output from Java or other plugins). It would be nice to have a keyboard shortcut for undo too.
Rating as 2-star due to the above bug (which was driving me nuts), otherwise it would be a 4-star review).