Could you share a screenshot?

I guess previewing images in iOS Photos app offers more control.

Sure. First you see the vertical orientation of an 800x1600 image. No issues.

Then I turn the phone horizontal. That's the next image. You can see that top of the is clipped, which seems like a minor bug itself, since it necessitates scrolling the image down to see the top (apparently the sizing algorithm doesn't take in to account the address bar?)

The third image is after the image has been scrolled down.

The final image is during a two-finger "squeeze" of the image: this is the kind of scaling I want to be maintained the same way that a zoom is maintained, but because the image doesn't fill the horizontal width, upon release of the two-finger squeeze it is immediately scaled back up to look like the third image. Doesn't make sense to me that this would be the behavior.

The image can be trialed at http://caseyconnor.org/pub/image/800x1600.png

Thanks. I am able to reproduce it. You're right. Your test image shows up just fine in Safari, but not in Firefox. It's a Firefox bug on Firefox for iOS scales images in Portrait and landscape mode.

I do recommend to open a a bug report. Simply send send your comment via email to ios-feedback@mozilla.com or go to Firefox > Settings > Send feedback. Even better open a bug report on https://bugzilla.mozilla.org

Finally got around to reporting a bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1637436

Edit: now being tracked here: https://github.com/mozilla-mobile/firefox-ios/issues/6608