Hi   !
Type in the adress bar   about:config (promise to be careful, if asked) Type and look for the preference : layout.spellcheckDefault and you can set it's value to either : Spell Checker Off :   0  (zero) Check Only Multi-Line Input Boxes :   1 Check Multi and Single Line Input Boxes :   2
You could also check the   spellchecker.dictionary

Some websites use their own spellchecker. Check your options on Yahoo.

Did you right-click in the editor area to see whether "check Spelling" has a checkmark and that a dictionary is selected?

Thanks for all your help so far, I tied the 'config' and that was set correctly. Yahoo does not have its own dictionary/spell check as far as my research tells me and the ABC and right click options do not appear. Therefore I have gone to Chrome for the time being but will keep everyone up to speed as I suspect it will just start again as quickly as it stopped!

Dont't know if it will help, but I found this article :
https://www.lifewire.com/check-spelling-yahoo-mail-1174517
And maybe, just to make absolutely sure that you haven't overlooked anything, you'd like to go over this article (again) :
https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker