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It's probably a matter of setting up the right environment for the userscript to run in. Currently it appears to work fine in:
Chrome + Tampermonkey Beta (5.1.*)
Chrome + Violentmonkey
Firefox + Tampermonkey if you set the TM Content Script API option to "UserScripts API Dynamic"
Has been very unreliable lately. In album/photostream/group pool view (thumbnails) it sometimes gets stuck at "getting original link" phase. In single photo view it often doesn't load the extra buttons at the bottom to open different resolutions.
Can you please fix those issues? I've been using this script for years, it's awesome when it works properly.