OK, after an almost infinite amount of faffing around I can finally send this using Watefox without red underlines : colour vigour humour rigour etcour (OK, not the last !).
And with red underlines : color vigor humor rigor etcor.
Don't ask me how many of my steps were unnecessary, but the last one (and therefore the one that
appears to have made the difference) was to install british_english_dictionary-1.19.1.1-typefix-sm+tb+fx.xpi
And then right-click in the "reply" dialogue (

) box and select Languages > English (United Kingdom).
** Phil.