Weird problem with png transparency in Firefox, black instead of alpha?

Question

css

I don’t know why, but lately the browser has become somehow strange to display the animated disappearance / appearance of png images with an alpha channel. Transparency is replaced with black. In the middle of the animation, the images look like this

Answer 1

On many sites? or on the one you create?

Answer 2

The problem was suddenly solved with the release of 12 fox