Add download button to Twitter image, and click to download the original image named by format.
This bug can't be reproduced on my device, might be caused by conflicts between scripts.
If you can provide your console log, the reason might be found.
Ohhh, I figured it out by looking at my console log (I should have thought of that). My Firefox privacy settings were blocking the user script from doing a cross-origin request, which is why the download failed.
It'd be nice to figure out how to make that stop happening, without going all the way down to "Standard" privacy protection, though.
Something seems to have gone wrong with this script in the last day or so. The icon is there still, but clicking it has no effect. Maybe Twitter changed their URL format for the full-size images?