דרדס משתמש Firefox‏ 15028044 מאת

Double click your German nouns to know their Der-Die-Das.
You can also search manually from the extension's icon.

The extension uses the de.wiktionary.org REST api in order to get the articles.
Here is the source code: https://github.com/kamomil/DerDieDas/tree/ff
Please give positive reviews if you liked it and report bugs in github.
enjoy