Until a couple of days ago my spell check worked fine but now it queries all plurals and other words that are ok eg "in parts at least had to be retyped" it marked parts and retyped. It is Thunderbird version 60.4.0 and in the Add-ons it shows and in Add-ons it shows British English Dictionary (Marco Pinto) 2.69 which I added to try to fix this.

I am pretty basic at computer skills!! Thanks Richard