Rated 4 out of 5 stars

Nice addon - a couple suggestions though
1. Firefox uses white as the default background color, but screenshots use black as the background (or transparent if PNG). Save images with a white background if none is specified
2. Save PNGs in PNG24 format - there's no need for transparencies in screenshots and the files will be much smaller
3. Increase JPG compression. I had full-page screenshots (somewhere around 1024x1800) as big as 1.2MB.

This review is for a previous version of the add-on (1.0.0.10).