hello, you can install the dictionary from https://addons.mozilla.org/firefox/language-tools/ - it should work without an issue...

Do you have a Windows account and a user name with non ASCII (accented) characters? Make sure that you have updated to Firefox 23.0.1. You can find the full version of the latest Firefox 23 release in all languages and for all Operating Systems here: *http://www.mozilla.org/en-US/firefox/all.html ---- Verify that the extension is unpacked and that you have the two dictionary files (de-DE.aff and de-DE.dic) in the dictionaries folder in its folder in the Extensions folder in the Firefox profile folder. You can use this button to go to the Firefox profile folder: *Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder) *http://kb.mozillazine.org/Profile_folder_-_Firefox If the dictionary doesn't work then try to move the two dictionary files (de-DE.aff and de-DE.dic) from the XPI (ZIP) archive to the dictionaries folder in the Firefox program folder (you may need to create this folder). * http://kb.mozillazine.org/Dictionaries