This one actually works! I've tried several other similar add-ons, and none of them worked. But this one does.
If it had the option to clear everything when I close Firefox, then I'd give it five stars.

It's not really worth using as long as it cannot clear data from YouTube, which is owned by Google, which is the largest data gathering company, and the one we'd most want to remove tracks from. Perhaps in the future this will be fixed and I'll give it another try and update this review. Good concept and a good start, but so far it's still incomplete.