A basic test with Firefox 57:
1) Color? OR Colour?
2) Favor? OR Favour?
3) Favorite? OR Favourite?
Bizarrely, while typing this I get squiggly red underlines on both "Colour" and "Favour" but NOT on "Favourite"! But also, "favour" seems acceptable, and as for "colour"? That works, too. What kind of English dictionary allows a word in lowercase, but not capitalised? -[ uh-oh, that wasn't recognised either! (but "recognised" was!) ]- with initial caps?
Perhaps Firefox remembers my saved spelling for "Favourite", from my using "Add to dictionary"? Well, to test the theory, let's add "Favour" ... that worked. And "Colour" …? Yep, that worked, too. Sigh. Guess I'll have to add capitalised (yes, that's the right spelling!) words to this dictionary individually …!
Cameron, where are you when we need you?
Also, a tiny peeve: Why wouldn't an Aussie spell-checker mark the American spellings wrong? I can think of two likely answers: (1) That might be too hard, depending on Mozilla's implementation of language variants (say they might allow everything in the "main" language dictionary, and ALSO anything in the "variant" or "dialect" dictionary) - which would be a pity, since we then end up with a Not-Quite-Right spelling for our variant of the language; OR (2) They aren't wrong - just less common. Historically, different Australian States taught different spelling systems in school (e.g. NSW used "-or" endings while VIC and TAS used "-our"), tho' I believe that may have changed with the common curriculum reforms. But in that case, a standard reference, such as the Macquarie Dictionary of Australian English, could be used as arbiter.
Mozilla, PLEASE let's have language variants properly supported by their individual dictionaries having primacy! E.g. en-GB should allow only those spellings used as standard in Great Britain; likewise for Australia, Canada, New Zealand, South Africa and anywhere else that officially recognises an English-language variant. Also applies to other language groups and their variants.