(Maybe an obvious trick, however, this was a surprise for me)
To pause GIF animation in Firefox, hit 'Esc'. Any animated piece of
art crap on the current page will stand still until the page is reloaded. Here is a sample to test:
More radical solution is disabling GIF animation at all in about:config:
image.animation_mode = none
Default is normal.
And I wish some other shortcut, say, 'Ctrl+Esc', would remotely annihilate anyone in the world with animated userpics. Click -- and gone. Dreams, dreams, dreams...
Inspired by: http://ffextensionguru.wordpress.com/2007/10/30/tip-pauserestrictdisable-animated-gifs/
Bug pic from http://dump.iof.ru/