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

I also use Grammarly for Firefox 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.

Do you have a dictionary installed and selected?

It is possible that a new default-release profile folder has been created. See "Determine if Firefox has created a new profile":

• https://support.mozilla.org/en-US/kb/recover-lost-or-missing-bookmarks

You can open the about:profiles page via the location bar to check whether multiple profiles are present and what profile Firefox currently is using.

• Do NOT click any Remove button on this page.
• https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

Firefox 67 and later can use a dedicated profile for each of the Firefox update channels and will lock the profile, so it can only be used by Firefox in a specific installation folder. For Release versions this profile would be a xxxxxxxx.default-release profile, profiles used in older Firefox versions usually used a xxxxxxxx.default profile (a refresh adds a time stamp).

• https://support.mozilla.org/en-US/kb/dedicated-profiles-firefox-installation

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 you need a dictionary and not a language pack that provides string translations for the user interface.

The recent update had removed my Add-ons .... so no dictionary. Put it back and now OK.

In FF 71.0 the spellcheck in Danish, marks ending of words that contain danish letters æ ø and å as misspelled - the part after the danish letter is marked, even tho the word is spelled correct, and is in dictionary (was before v.71.0 anyhow...). Sometimes switching to English language and back to Dansish removes the markings.