Basically, it's the Firefox engine, with a different body, DG. The latest version seems to be spot on, and runs significantly faster than the last one. One thing some might find useful is that it
doesn't auto-update, but checks periodically, and then asks if you want to update if one is available. Also, many (but not all) Firefox add-ons work on SeaMonkey. One thing I would say is, the default theme is horrid, but Modern is also pre-installed, is much nicer, and is available simply by clicking View > Apply Theme. It's also much quicker at starting up than any version of Firefox I have used.
http://www.seamonkey-project.org/