Work on my Firefox 57.0.1 x64 Windows 10.
This extension does the job of 'ImageBlock' Add-on and Google Chrome's per-site permission settings (for images).
Learn to use it slowly. This is a great Add-on but the UI can be confusing.
For dev, I suggest to:
1. Put the white & black list in the separate option page.
2. Make separate switch (to block / load images) for global (all sites) and the currently opened site*.
* then automatically added to white & black list.
** or you can put per-domain setting in the domain list and make the switch in toolbar icon only for global setting.
Thank you.