The first version of Firefox was released in 2004 and since then, some 230 million people world-wide (or 20% of the 1.2 billion strong Internet population) are using Firefox as their primary web browser.
The next graph illustrates the Firefox development process at Mozilla. A core set of 400 developers are responsible for the code that runs Firefox while the rest is a community driven effort. For instance, there are around five hundred people, outside the core developers, who localize Firefox into over 60 languages.
It’s a visual guide (with plenty of screenshots) to help you learn everything about Firefox browser from using tabs to downloading files to using themes & plugins. The manual is under "free documentation" and also available for download as a PDF ebook.
As power users, you probably know most things that are mentioned in the Firefox manual but you may still want to pass a link of this manual to someone who has just switched (or is planning to make a switch) to Firefox.