I finally remembered to change the dictionary Firefox uses to check my spelling. I am not sure at which point Mozilla decided I wanted the US-English version installed, but I am hoping I will start to look like I know when to use an S or a Z in certain words. Harmonise and harmonize. Arrrgh, no, neither of them has some red underlining. However less debatable words and spellings are highlighted nicely. If I type in faucets, Firefox refuses to offer any alternatives at all, meaning I have to remember to use the word tap all on my own! Sidewalk triggers the same spell check alert, so I’m guessing that S and Z have become more interchangeable than I realised?