Image properties shows displayed width and height, not image's actual properties

Binary package hint: firefox

Repro steps:
Set your preferences to zoom out oversize images (can't find it in Preferences, is it always on? I recall it used to be an option).
Navigate to an image which is larger than your current window. Your cursor should display as a magnifying glass with a plus sign inside, indicating that clicking on the image would zoom it to 100%.
Instead, right-click on the image, and pick "Properties".

Actual result:
The "Width" and "Height" properties are those of the zoomed-out image you see on your screen, not the actual image's properties.
For comparison, zooming in and selecting Properties again displays the image's real properties.

Expected result:
I would have expected the image's native dimensions. That would be consistent with the file size field, which shows the size of the actual image data, not whatever the zoomed-out image turns out as in memory (at least the byte count is identical to that in the downloaded image file, for one I tried).