This is a list of the best add-ons for German users. If you would like to suggest an add-on for this list, feel free to email me at fwenzel (at) mozilla (dot) com.
Blocks annoying video ads on YouTube, Facebook ads, banners and much more.
Adblock Plus blocks all annoying ads, and supports websites by not blocking unobtrusive ads by default (configurable).
ImTranslator provides access to the online translation powered by Google Translate, Microsoft, Babylon for 66 languages. It includes dictionary, text-to-speech, back translation.
Complete Video Tutorial: http://youtu.be/UwK6529XZIQ
German Dictionary (new Orthography) for spellchecking in Mozilla products
Get international weather forecasts from AccuWeather.com and display them in any toolbar or statusbar with this highly customizable and unobtrusive extension.
This add-on likely is only useful for users who speak German or visit German language websites - it will remove so called "Binnen-Is" on webpages.
This Add-on shows water levels of German Rivers.