Before there were dozens of decent ad-blocking extensions out there I was a big user of Privoxy. It had an action called "deanimate-gif" that would display only the first frame of the animated image.

I'm not sure if Privoxy was downloading the whole image in the background in order to extract the first frame, and I don't know if deanimation used less CPU or RAM than just displaying the animated versions, but I loved the feature regardless.

If you're willing to get rid of all gifs, toss this in your user stylesheet:

    img[src$=".gif"] { visibility: hidden; }