Good, but... Rated 3 out of 5 stars
I think that a tool such as this is better suited to be a stand-alone than a browser Add-on. I would suggest making this tool separate with XULrunner. As it is, though, I would like the ability to convert GIFs, import multiple files at a time, and open APNG files back up.
To Prospective Users: it hogs memory, and is not for the faint of RAM nor the weak-CPU'd. It takes a LOT of RAM, even for just 15-20 bite-size frames, it can take up a lot of your RAM.
Despite all this, I think this tool has great potential, and APNG itself has even greater potential. I hope that APNG will get more support soon, like from Adobe. I will try to support APNG, and I plan to use it if I ever need to use animation on the web w/o Flash.
Overall, 3 stars. Get more features to get 4 stars. Get APNG supported by Adobe (esp. Photoshop) and another major Web Browser (IE (Yeah right), Opera, Safari, or Chrome), and that 4 will become a 5.