Just select the English UK dictionary, or whatever you usually use, instead.

In Windows & Linux that is by right clicking on a misspelled{uk) misspelt {usa) word and selecting from the languages offered. I assume it is the same or similar on a Mac

P.S

• See How do I use the Firefox spell checker?_switching-dictionaries

On a Mac it is Ctrl click instead of right click, you will not have a right click button.

Aidanohara said

I write my gmail messages in English. Since yesterday the words are all showing they require correcting (red dots underlining). I don't know why. It may be because I may have selected Firefox's Irish (IE) Language Pack (dictionary) recently.

Thanks, John. Problem solved.