Pause/Restrict/Disable Animated GIFs
Adds control over animated GIFs

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Tweak Type:	Display
Fx 2:	YES
Fx 3:	YES
Fx 3.5:	YES

Here is a handy tip for dealing with a site that has animated GIFs, pressing 'ESC' key will pause animated GIF's on a page. This works in both Firefox and Internet Exploiter.  Note: in order to 'resume' the animation you will need to refresh/reload the page. You can test this out with the image below:

Additional options via an about:config tweak:

1. In a new tab type about:config in the address bar and press enter
2. In the filter field type animation
3. In the results listed double-click on image.animation_mode
4. In the pop-up box type in one of the following:
   • none — will prevent image animation
   • once — will let the image animate once
     
   • normal (default) — will allow it to play over and over

Note: 'Once' setting does not appear to work in later builds of Firefox 2 and does not work at all in Firefox 3.

Source