Rated 5 out of 5 stars
This add-on is brilliant. I was looking for something like this, and, by coincidence saw this on google. I created an account on Firefox Add-ons just to download this.
So far it seems to be working fine. Although my browser seems a little jumpy, that may or may not be due to the add-on (I think it's actually due to the mozilla add-on site). The add-on does what it says, and from a few minutes of use, seems to be very useful, not only for learning and improving mandarin, but for general understanding of untranslated pages as well, just by putting the mouse over mandarin words. I'll try this program out some more, and I might come back to make more comments. Otherwise, it is very nice, although maybe a bit buggy (maybe).
Many thanks to the creator of this add-on.