Comment by Worldblender

It does use the same engine from Firefox, although not always the latest version. This project strives to backport security updates from newer Firefox versions whenever they can, sometimes being able to upgrade the base Firefox version, while keeping the legacy XUL based add-on support. This is actually one thing I like wtih Seamonkey, alongside its HTML editor (that which could see better support for CSS and Javascript/ECMAScript stuff) and GUI based image blocking.