Do you see this extension listed under about:addons -> Dictionaries?

Note that dictionaries need to be unpacked in order to function properly.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

The dictionary is added in

about:addons -> Languages

I attach screenshot.

I didn't find the word "Dictionaries" in about:addons.

You installed a German Language pack not a Dictionary.

After installing some extensions, you have to change your preferred language to (example) Canadian English by right-clicking on a text box. (Any text box) This adds spell check functions for (Canadian) German.

There are several listed here : https://www.google.ca/search?q=firefox+german+dictionary+plugin&ie=utf-8&oe=utf-8&gws_rd=cr&dcr=0&ei=PLMIWr6jI4ucjwO05YuQDg Make sure it is version 57 compatible.

Once installed it will be in Plugins: Dictionary You will also still need Spell Check ticked.

Note : Firefox 57 Quantum release date Nov 14th https://www.mozilla.org/en-US/firefox/quantum/ Note : Legacy Extensions will be disabled and or removed.

Please let us know if this solved your issue or if need further assistance.

Note that language packs add translated string values and do not include the dictionary.

You can look here for dictionaries:

• https://addons.mozilla.org/en-US/firefox/search/?q=german%20dictionary