brilliant Rated 5 out of 5 stars
I have been waiting forever for this. This is a bug if you ask me and should be fixed in Firefox itself, but hey if an add-on fixes it then it's the next best thing. It works perfectly. I moused over an image, saw the alt-text and it was still there when I came back several hours later!
Furthermore, there so far appear to be no unwanted side-effects.