Firefox is not very fast, nor light. However, it is pretty solid, IMHO, and it has very good extensibility & customizability with the help of enthusiatic users.

BTW, both Firefox and Mozilla are based on the Gecko engine, thus, they have a lot in common. However, what I don't like about Mozilla is its similarity to Netscape, which IMO was not a well-done piece of software. Firefox (used to call Firebird), on the other hand, is a lot lighter (or you may use Thunderbird too, if you prefer using the engine for e-mailing as well).