According to Apple you shouldn't have any other FW devices connected, though they don't mention USB.
Firefox prefs are in user/library/application support/firefox
You may want to take out the whole folder and put it in the same place on the new system once you install FF.
Serial numbers and passwords can be in many places, as plists in your user/library/preferences or sometimes within the application itself or the afformentioned Application Support folder. Be prepared to do some digging.
Hey, it happens to most of us at one time or another.