They marked it as a duplicate of the given bug, and a mozilla security manager politely explained why:

> The "documentation"(?) was maybe an announcement about the changes in bug 754608, which made the thumbnails follow the caching rules for https. Https pages can indeed be cached depending on whether or not they're marked "no-store". bug 755996 (and you, in this bug) says that's not good enough which may be true, but is already covered in bug 755996.

... Which seems to be a fair explanation of the situation. HTTPS pages that are cached can still cache their thumbnails. I'm still not seeing the outrage-worthy malfeasance here.