I wanted to add the german dictionary for spell checking. After finding the right dictionary and adding it, the german language didn't show in the spell checking menu. If further details and screenshots, please let me know. And thank you in advance.

So installed the Dictionary and not the Language pack from https://addons.mozilla.org/firefox/language-tools/ ?

https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker

Did you read these instructions: https://support.mozilla.org/en-US/kb/use-firefox-another-language

Did You read this article?. This article is for how to use Language Packs and not Dictionaries as it has nothing to do with the Spellchecker in Firefox.

I downloaded this dictionary: https://addons.mozilla.org/en-US/firefox/addon/deutsch-de-language-pack/ and I have tried out these instructions already and didn't help.

I downloaded this dictionary: https://addons.mozilla.org/en-US/firefox/addon/deutsch-de-language-pack/ and I have tried out these instructions already and didn't help.

This is a Language Pack and not a Dictionary.

from on top of https://addons.mozilla.org/firefox/language-tools/ (Dictionaries are on right column)

Installing a dictionary add-on will add a new language option to your spell-checker, which checks your spelling as you type in Firefox. Language packs change your browser’s interface language, including menu options and settings.