Seems to be resolved by updating Waterfox from 55 to 56.
@TeamYouTube I recently enabled multithreaded mode on my Waterfox browser and found out that this (and not any addons, I tried disabling all) prevents me from uploading video thumbnails. The upload doesn't transfer the selected file and the progress indicator spins endlessly.
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Thanks. Twitter will use this to make your timeline better. UndoUndo
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