From MozillaZine Knowledge Base
You can change image animation behaviour in the following ways:
Image animation preference
The image.animation_mode preference determines how multi-frame GIF images are animated. This preference settings affects GIFs in every kind of displayed content, including web pages, messages and RSS feeds.
You can change this preference setting using any of the usual methods for editing configuration, specifically:
In Mozilla Suite and Firefox:
In Thunderbird, use about:config and filter it for animation to find the preference.
Pausing animated images
Pressing the "Esc" key on Windows (other OS may be different) will pause animated images on the current website. Note: Starting in Firefox 20, this is no longer the case due to the fix for bug 614304 unless resolved in an upcoming patch (see comment 45). The SuperStop add-on by Gavin Sharp is an alternate solution.  
The following programs have options that can turn off animation on your system:
This feature must be turned on or off from inside the aforementioned programs, not in Firefox, Mozilla Suite, or Thunderbird.