Based on https://bugzilla.mozilla.org/show_bug.cgi?id=1381084#c3 I think we need to step back and figure out if there are some ways worth trying to set network load priorities early enough on image loads. I think an interesting metric could be to figure out how many images that end up not rendered in the view port are loaded (onstoprequest) BEFORE images that do. If it's a negligible amount (because on modern sites <img> presence in the markup somewhat reflects screen priority - from top to bottom), I don't think we need to do any deeper optimizations.

I have some different testing plans (more local-analytical), hence a wide deployed telemetry is not urgent to have soon.