It is highly standards compliant, supporting many standards from bodies such as the W3C, IETF, and ECMA.
Mozilla is built on a very big source code base and is far larger than most Open Source projects. It is 30 times larger than the Apache Web Server, 20 times larger than the Java 1.0 JDK/JRE sources, 5 times bigger than the standard Perl distribution, twice as big as the Linux kernel source, and nearly as large as the whole GNOME 2.0 desktop source—even when 150 standard GNOME applications are included.
As a result of the browser wars in the late 1990s, the platform has been heavily tested and is highly optimized and very portable. Not only have hundreds of Netscape employees and thousands of volunteers worked on it, but millions of end users who have adopted Mozilla-based products have scrutinized it as well.
-- Rapid Application Development with Mozilla, Nigel McFarlane (2003)
Also, a thing or two about Firefox add-ons development:
Mozilla Developer Network (MDN)