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).

What sort of problem are you referring to?

It is not Firefox's fault, but Huasing's. I'm now working onto remaking all the Huasing pages, one by one, to get rid of features only for M$ IE.