On Firefox when you go to the full image you see it being drawn over a white background

[reidsci]

Steps to reproduce the behavior:

1. Go into Firefox in private mode. (Each time you enter private mode the cache starts out empty so you are not grabbing cached copies of the image!)
2. Go to https://fancyapps.com/fancybox/3/ and click on one of the images so that fancybox brings it up full size.

Expected behavior
The expected behavior is that the originally clicked on smaller copy of the image is replaced by the full size version as it is being loaded. Instead there either is a white flicker or white flash, or in the case of a slower loading image, an unscrolling of the image over a white background. In the fancyapps.com demo page linked to above, it is a flicker. On a site served images by AWS S3, it may be a 1 second or so unscrolling over a white background.

Screenshots
Example of the problem taken from my own website:

Desktop (please complete the following information):

• Firefox 89.0.1 (64 bit))
• Windows 10

Additional context
This is a Firefox only issue.