Usable, stable but a small privacy issue Rated 4 out of 5 stars
An add-on with which one can create APNG files and don't get mad: in this respect it's really more usable than the "APNG Edit" add-on (that said I'm very respectful for all the work done on "APNG Edit"). Animat is just better.
I use Animat with Firefox 3.6.x and haven't had any stability problems.
Note that there is a small security issue with the versions up to 1.04 (included): the .animat format uses the full path on the filesystem of PNG images used, thus disclosing the user's name and an existing path on the host system. So don't publish your .animat files on the web until a new version that fixes this problem.