To add a dictionary:
Go to https://addons.mozilla.org/en-US/firefox/ and choose “Dictionaries."
Click “Install” next to the appropriate language.
Click “Restart Firefox.” If nothing happens, quit Firefox manually and reopen it.
To change between dictionaries:
Return to https://addons.mozilla.org/en-US/firefox/
Search for “Dictionary Switcher.”
Click on “Dictionary Switcher.”
Click “Install Now.”
Click “Restart Firefox.”
At the bottom left corner of the Firefox window it will say “auto: en-US.”
Left-click on "en-US" to cycle through the language options.
Right-click on "en-US" to simply select the language.