Bug 650968 kind of did this half-way: once we apply the cropping, we lose the animation (since we just crop the first frame). But there are still cases where we show the animation (e.g. during preview, before a restart), and so that leads to inconsistent behavior (bug 819190).
I'm not sure what the best way to do this is - perhaps a CSS style that prevents an animated image from being animated?
How about cropping the image right after installing?
That doesn't help with preview though - we can't do cropping on the fly like that. I spoke to #gfx about this and Seth suggested that it may be relatively easy using the temporal media fragment he has been working on. I think he was going to file a bug about that and land it relatively soon.
If we could add a CSS property to disable animations, that would probably be incredibly useful to certain extensions.
I've opened a bug for supporting temporal media fragments for animated images at bug 821571.