Very useful to have the image larger without having to click through. Sometimes this addon breaks when there are image links all next to each other (like on Bing or Google) and will just immediatly open the image in the preview making it impossible to click anything under it. The fix is to hold Ctrl to prevent it but it's still annoying.

I would still recommend this addon though.

Doesn't work the majority of the time. I am really annoyed that this doesn't work at all the majority of the time and it just keeps getting worse. Only around 20% of the time it works. I always keep either getting an orange spinner (e.g. Facebook profile pictures from the Newsfeed) or nothing happens. The extension is really flawed.

Dfff

Check permissions -- "Access your data for all websites".
According to support.mozilla.org, this includes user names and passwords!