Which dictionary are we using? (Or are we allowing English spelling to be supplanted with American spelling?)

Which dictionary are we using? (Or are we allowing English spelling to be supplanted with American or other spelling?) Whereas in MS Windows I can choose UK English or one of the many different forms of English in use elsewhere, I'm noting that in Mozilla/Firefox I get the typical warning under a spelling that does not equate with the spelling used in the current dictionary and yet these spellings are not UK English... Even I choose UK English as my default dictionary the spelling of "colour", "neighbour", aluminium, still give this indicator.