As of today, I am no longer maintaining or supporting the Next Image extension for Firefox.
I don’t use Firefox anymore and, honestly, it’s just too much effort supporting a Firefox extension. Also, most of the functionality in this extension can be accomplished with a bookmarklet.
I have posted the code on github for anyone who is interested in reviving this project. However, most of the effort is administrative. Thanks to everyone who used and reviewed Next Image. Hopefully you will find an acceptable alternative.
Update Here’s a bookmarklet that works well as a Previous button.