Version 111.0buildid20230309.232128 fails to mark error prone instead of error-prone.
Some modern words like playtest can be missing!

As I am on holiday in Gambia I think that this confused the works hence 5 *only.

Bien

How do get this to work with the spell checker? That only looks at US "English"

Doesn't work. Switched to cybrkyd's pack and it works instantly. I admire Mozilla for challenging the Chrome hegemony, and always stick with Firefox despite it's many, many flaws, but damn is it hard to love them sometimes. Constant own goals and strange blunders.

Anyway, use cybrkyd's pack.