When you think of free Firefox eBook, I would like to think that your first thought is Firefox Facts. I have found another one for you though, and this one has more to do with Greasemonkey.
Dive Into Greasemonkey is a book about programming with Greasemonkey, a Firefox extension for customizing web pages. Read it online for free.
It seems like lately I have been getting more interested in Greasemonkey scripts, so reading this book should be a lot of fun. Go pick up your own copy or read it online at DiveIntoGreasemonkey.org, or read it right here (check the box bellow).
Need Help? Be sure to check out the Beginner’s Guide to Greasemonkey!