Firefox Dictionaries

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.