Type about:config<enter> in the address bar. If a warning screen comes up, press the I Accept the Risk button. At the top of the screen is a search bar.

Type: layout.spellcheckDefault Its values are; 0 Spell Checker Off 1 Check Only Multi-Line Input Boxes. 2 Check Multi And Single Line Input Boxes.

Also, check the spellchecker.dictionary entry.

https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker

Grammarly for Firefox by Grammarly https://addons.mozilla.org/en-US/firefox/addon/grammarly-1/

Grammarly will help you communicate more effectively. As you type, Grammarly flags mistakes and helps you make sure your messages, documents, and social media posts are clear, mistake-free, and impactful.

Thanks for the quick response :-) I had done everything apart from trying Grammarly - it's a pity something similar isn't built in by default and that you have to sign up separately for, it but now all of the context menus and red underlines are shown :-) Thanks Again!!! :-)

Do you have a dictionary installed and selected?

You can see which dictionary is selected if you right-click in a text area and open the Languages sub menu.
At least one dictionary should be installed and have a check mark to indicate that it is selected.
*open the "Add Dictionaries" link to install a dictionary if you do not have one.
The spelling checker is enabled if [X] "Check Spelling" in the right-click context menu has a checkmark.

You can look here for dictionaries:

• https://addons.mozilla.org/firefox/language-tools/

Note that for spell check you need a dictionary. A language pack provides string translations for the user interface.

• https://support.mozilla.org/en-US/kb/use-firefox-interface-other-languages-language-pack

You can check on the about:config page that layout.spellcheckDefault is set to 1 (multi-line) or 2 (single-line).