Let's try this :
Type in the adress bar   about:config
(promise to be careful, if asked)
Type and look for the preference :
layout.spellcheckDefault
and you can set it's value to either :
Spell Checker Off :   0   (zero)
Check Only Multi-Line Input Boxes :   1
Check Multi and Single Line Input Boxes :   2

Is the problem with one language? What about the others? What is your primary language on Firefox?

Thank you for helping me.

Happy112, I did what you're suggesting already yesterday, as I read it in another question, and I set it to 2. I just checked, it's still like that.

FredMcD, none of my languages are corrected. My Firefox is displaying in English.

Your system details show that you have some language packs installed,   but have you considered installing a dictionary in the language(s) of your choice from here :

https://addons.mozilla.org/en-US/firefox/language-tools/

That solved my problem! The missing dictionaries. I don't know why but I implied that a language package was including the dictionary...

I'm wondering why these things are not explained in the page https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker?

Anyway, thanks for your help.

AudeDL said

That solved my problem! The missing dictionaries. I don't know why but I implied that a language package was including the dictionary... I'm wondering why these things are not explained in the page https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker? Anyway, thanks for your help.

You've got a point there   -   should be explained in that article. But I sure am glad that your problem is solved now   -   enjoy your
dictionaries   !