Arctic Warrior: try the off/on routine:
uncheck animation
save settings, quit browser
start browser, check animations
well?

for firefox, I don't see either where it is accessible from the menu. You can change it by editing your prefs.js file. the key is:

// Image animation mode: normal, once, none.
// This pref now has UI under Privacy & Security->Images.
user_pref("image.animation_mode", "normal");

there's a lot of info in http://www.mozilla.org/unix/customizing.html

(I use Linux, but I think it's the same for windows)