SeaMonkey is an offshoot of the Mozilla project (similar source code) and is actually a suite (browser, email client, IRC, etc). It is community developed like Firefox, so has frequent updates and has Nightlies. Extensions are available, but not all that many.
The good thing is those that develop SeaMonkey have never really screwed with the basic premise of what a browser should be. In essence, a retro-browser.
http://www.seamonkey-project.org/