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In Firefox 66.0.1 (and for a few versions by now), many user icons fail to load on Stack Exchange pages. For example, on this question, the asker's icon and the icon for the writer of (currently) the only answer fail to load, while with Jenayah, the most recent editor of the question, her icon loads.
This problem doesn't occur on Chrome. Right-clicking and reloading the images doesn't work; viewing them in a new tab shows that the browser is able to display the files. This may be a problem with Stack Exchange's inner workings cooperating with Firefox, or something may just be wrong with my download of the browser.