Doesn't work in later FF versions! Rated 1 out of 5 stars
As already pointed out by "Equaer", the default "Wipe Type" can no more be picked in the "Options" of later FF versions. Also, the little blue progress bar that appears during the wipe operation is so fast that it's safe to say that no wiping takes place, but just a simple deletion.
I tested also on an old FF 3.5 version, and there wiping seems to work, as the progress bar takes some time to progress (as it should), depending on the amount of data in the cache.
So, in effect, in later FF versions, the add-on does NOT wipe data, and it's useless as long as it doesn't get updated. Only 1 star this time.