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

There are both Language Packs which change the language of the user interface, and Spellcheck Dictionaries available there. To do a text area spellcheck you need the Dictionary.

In case you are curious about how Language Packs are used in Firefox https://support.mozilla.org/en-US/kb/use-firefox-another-language

I found out by myself, thank you all for giving general instructions already available. The real solution was: 1- Remove the language package already installed from the right mouse button 2- Do not use the right button to install the language package!!! It leads to a faulse impression that it is installed, but it isn't! But it tells you that it is when you go and check in your add-ons. '(Programmer mistake)' I hate that, many programmers leave dead end routes, even in major cities sites! Like Montreal. Programmers are suppose to have many different routes that works and achive the same objective! That's what we call USER FRIENDLY, humans are all different, we don't think the same way! It's the programmers job to make it easy, and remove dead end routes that gives a faulse impression that you did thing correctly! 3= Use the route: Select 'Add=On' option, not evident, lauguage package are not offered, you have to do a language search within the add-ons, then select your package ans install