replace embed iframe, object with anchor link.
This is probably a conflict with other addons: https://github.com/greasemonkey/greasemonkey/issues/2524
However I don't think there will be a fix for it. Try removing refcontrol or noscript addon if you have them.
Thanks, that does appear to be the cause. I use Privacy Badger and disabling it in the addons list gets the images loading again (I'd previously only tried its 'disable privacy badger for this site' option).
Bug? Cannot load image URLs
Trying to open an image URL just gives me a blank white page in firefox 54 on linux, I don't have any other userscripts running and disabling this one fixes the problem.
Example URLs:
https://i.imgur.com/EaGVFtI.jpg
http://kentuckyroutezero.com/newsletter-sep2014/MadeForVGA/KrzAct1.jpg