Looks like FLIF can do animations as well. http://flif.info/animation.html
And I dont know of any popular project that uses MPL apart from Mozilla Firefox.
Eigen (perhaps not "popular" unless you're interested in doing linear algebra with C++.. :-/ )?
Surely there are others as well..
If you want a weak copyleft license, IMHO MPL-2.0 is about the best available at the moment, and relatively easy to follow.