A different take on Google direct image viewer
< Google Image Direct Viewについてのフィードバック
This could also be an issue with Firefox Content Security Policy with respect to Tampermonkey. See open issue here.
Disabling CSP worked, but it is a risky thing to do.
Yes. I agree but that issue is out of our scope. There seems to be some workaround available in Tampermonkey BETA 4.11.6113.
Works with the TM beta workaround
Does not work
Clicking on the image opens the web page, just as without the script. Tampermonkey on Firefox developer edition. Only works if CSP is disabled globally.