A Firefox update apparently changed the private tab icon, and puts it on the tab count button on the toolbar.

It's tiiiiny, maybe a few mm tall, so the detail evident in this screenshot is not visible. As a result, my brain recognizes it as exactly one thing:

A bikini or halter top. With, uh, dots.

Which, given what humans are known to do with private tabs, seems a bit on the nose.

(I think it's supposed to be a mask with ears)