Very useful and works well (most of the time, so long as there hasn't been a sudden API update on the sites). A few requests I have would be:

1. Different modes of auto-fit for people with different preferences (in particular, auto-fit showing the control, auto-fit showing control and preview list, and auto-fit image only).
2. Preview list showing the current and previous image (not a big deal, it's just a preference).
3. Sankaku Chan and rule34.paheal.net. Not sure how to do it exactly, but Sankaku Chan does have a mobile app and the mobile app Nori managed to do Rule 34 Paheal.

The extension is fine as it is, although I noticed the Firefox update rate is slower than Chrome's, but it'll be nice to have the option.