FireFox Portable (3.6.2 right now) has an English dictionary but the compilers were not that diligent. eg favour comes out favor.

No. You can't say the compilers were not diligent. It's simply that you have a US English dictionary. Not wrong, just different. I find when I am updating Firefox it's almost invariable US English. They don't usually seem to have an British English version available. There may well be British English dictionaries available for Firefox, but I don't find it worth the effort of looking for one. I just add the UK spellings to the dictionary provided as the need arises.