Mozilla Firefox Spell Check

If the spell check facility in Firefox does not work try the following.

Go to options... General... tick 'Check your spelling as you type'.
Next to this you will see the option to choose your default language.

Mozilla Firefox Options Mozilla Firefox language

Having completed the above the spell check may still not work because the dictionary for your chosen language is not installed.
In my example for the British English dictionary go to:

Here you can:
+ Add to Firefox

All other language dictionaries are accessible from here.

Going back to the Firefox menu and selecting Add-ons should now show you your language(s) that are installed.