Some user avatars are now (again?) missing, why?

Starting a couple of days or weeks ago, for some user avatars on all Stack Exchange sites, only the alt text shows. This included my own, until I changed my profile settings to use Gravatar instead of a manually uploaded picture file. After I changed that, I can see my own avatar in all places I should see it, but some other users' avatars are still broken.

This could be Stack Exchange itself changing but it could also be my browser. Currently Firefox 153 and updates frequently, so it's what I suspect first. Still, anyone kind enough here to share their insight? Thanks in advance.