I don't necessarily see the need to change the cast of Firefly, as you cannot necessarily include everyone from every type of individual, or else you would need a cast of hundreds.
There are 4 Girls and 5 Guys, so it is gender equal. Even still, 3 of the Girls and 3 of the Guys look to posess 'strong' personalities, so it will not be a show where the guys do everything to save the girls.
There are 2 African Americans, 6 European Americans, and 1 Canadian. Try to pick out who?
To add an Oriental would be the next obvious choice, possibly a Hispanic, but would that really increase the demographics? I watch the show "Sex and the City", even though there are no lead male roles. I don't necessarily believe that every show must have equal demographics with its main cast. However, should the minor cast (i.e., eposidic cast or frequent guests) not represent a realistic demographic, then there is something up. You might not have a major character being White when you are dealing with an African-American family sitcom, but you do have reocurring characters who are.
I feel that Firefly will probably continue with the introduction of different cultures intermixing in space, hopefully not representing itself in a total historic context (like the US 19th Century west!!!!!).