It used to be a great add-on till they changed it all.
Now you don't have a visible index on the left side like you used to have, you can't just switch to another chapter that easy anymore or see where you are at, you need to to go to the index button first to make the index visible and if you want to further read your last page you have to close the index again. It is not possible to see the index and the page you want to read simultaneously. A sidebar would be way better than an autohide bar or at least the choice between those two and not just the autohide bar this add-on has now. I also can't just click on a epub download and it opens automatically in the epubreader, instead I have to save it, go to firefox open the epubreader and then search for my book which is really annoying and not user friendly.
The older version is great and much more usable, this new one has too much flaws.