Rated 4 out of 5 stars
Since this add-on doesn't work at all with 3.6.3 even after changing the maxVersion, I found a similar add-on so we can all highlight again.
It's called TextMarker!
Unfortunately that add-on also needs to have the maxVersion tweaked, it doesn't have a toolbar icon -- you have to either find it on the right click menu or you can use the "B,N,M key to highlight -- 3 different colors", unlike this one -- it doesn't give a warning and all highlights are gone once the page is refreshed; but it works and like this one -- it doesn't save/bookmark highlights on other webpages in case others use the browser or you don't want your privacy compromised. So use that till this is ever updated.