Right-click the textbox > Languages > English (United Kingdom). If you don't see it, make sure the [https://addons.mozilla.org/en-US/firefox/addon/british-english-dictionary-gb/ British English Dictionary] is installed.

Thank you.

''<p>adampembs [[#question-1445986|said]]</p>'' <blockquote>Why aren't you using an English (GB) dictionary? </blockquote> from https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker <blockquote>Not all locales come with a dictionary installed due to licensing reasons.</blockquote> If you are not using the en-US locale then you likely will have to install a dictionary. Note on https://addons.mozilla.org/firefox/language-tools/ the Language Packs are a way to change the language of the user interface like the menus as the spellchecker uses dictionaries.