...
If you do not have the language option then select Check Spelling and the languages option should appear
Following these steps will take you to a list of dictionaries that you can install and use.
...
After the dictionary is installed, right click on the word/text and open languages again.
You should now have a choice between the standard dictionary and the new installed one.
If it doesn't appear straight away, you may need to refresh your browser.
https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker