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.
