Generally speaking, is that Forth quicker than C?

Not generally, no. It can be, but that's not the point.

I hope to know, if we write OpenBIOS by C, is it quicker?

Possibly. But the project is about writing an IEEE 1275-1994 standard compliance firmware, not about booting as fast as possible. If that is what you're looking for, you should check out the coreboot project at http://www.coreboot.org/

If you're looking for IEEE 1275-1994 compliant firmware in C, you might also be interested in CodeGen's SmartFirmware. http://www.codegen.com/SmartFirmware/ and http://www.openfirmware.info/SmartFirmware

BTW, which program language does used to write FCode evaluator?

The OpenBIOS implementation is written in Forth.