I have 2 dictionaries in spell checker, English US and English UK. I have always had these, now for some reason when I select the UK version it is fine, but next time I open Thunderbird it has reverted to the US version. How can I stop this, or even delete the US version leaving the UK one alone?
http://kb.mozillazine.org/Dictionaries_(Thunderbird)
You can select the default dictionary, and add or remove other ones in Preferences / Settings. I can't point you to exactly where at the moment, but I'm sure it's not hard to find. Possibly under Composition in Settings?
Try edit, then preferences then click on the spelling tab. HTH.