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

You need to make sure you have the en-GB British English dictionary installed in Firefox and selected in the right-click languages.

Scroll down the list on https://addons.mozilla.org/firefox/language-tools/ English (British) English (British) Install Dictionary (379.7 KiB)

The Firefox version from the repositories of your Ubuntu Linux distribution might behave differently than the version from the official Mozilla server. Ubuntu has the tendency to bundle a lot of locales and dictionaries with its special branded version.

I'm not seeing any dictionaries listed under Extensions in the System Details list.

Did you check the Dictionaries tab in Preferences?

" .... Did you check the Dictionaries tab in Preferences? ..." HUH? I looked under preferences and so far have not found that tab.

Sorry, dictionaries are of course listed in the Addons manager (about:addons page).

see attachment - English - U.S. language pack incompatible with v 64. WHAT?

The Language Packs are for changing the language of the User Interface or UI of Firefox.

The language packs were either installed in the Package manager for the Ubuntu provided build of Firefox you have or you installed from addons.mozilla.org . Being incompatible means they are older versions for say Firefox 63.0.x and not the current 64.0 Release.

Besides your question was about the Spell Checker in Firefox and not the UI language. As I said in earlier reply you need Dictionaries.

Scroll down the list on https://addons.mozilla.org/firefox/language-tools/ English (British) English (British) Install Dictionary (379.7 KiB)

Look on the left as the Dictionaries panel is above the Languages.

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