There used to be one such Firefox addon that even made it into the Debian (Wheezy/Jessie) repositories. Unfortunately it was written for the old plugin system a

There used to be one such Firefox addon that even made it into the Debian (Wheezy/Jessie) repositories. Unfortunately it was written for the old plugin system and never updated and is now designated as blocking the new Firefox multiprocessing, so I already started looking into Firefox addon programming just to replace it. Somehow I never thought about looking for newer alternatives in the official addon store, where several modern ones seem to exist already, so I'd like to thank you a lot for your suggestion.