When opening a page with or without an account, I see message in the browser-console:
This page is using the deprecated ResourceLoader module "jquery.hidpi". Use of the srcset polyfill is deprecated since MediaWiki 1.32.0
Browser Console in (german localized) Firefox: Extras --> Webentwickler --> Browser-Konsole (in english maybe Extra --> Web Development --> Browser Console)
"jquery.hidpi" was introduced to Popups in https://gerrit.wikimedia.org/r/208515
It was deprecated in T127328 by https://gerrit.wikimedia.org/r/441614
Popups uses jquery.hidpi bracketedDevicePixelRatio provided by jquery.hidpi
We should use https://caniuse.com/#search=window.devicePixelRatio instead.
Quick test: Check that images inside page previews are still rendering as expected on https://en.wikipedia.beta.wmflabs.org/w/index.php?title=Special:AllPages&from=Asian+badger
Test on a retina device - images should be higher resolution.
(note if no retina device you can test this in Firefox by using about:config and setting layout.css.devPixelsPerPx to 2)