K-Meleon was a nice browser which disabled the image animation on a keypress. It's a beautiful browser with lots of options, but not the most compatible. There is a patched version 1.5.4 for Windows 98, that enables TLS.
For all retro epliepsy users: Mosaic from 1995 doesn't load GIF animations at all.
When image animation is disabled, the GIF will stay at Frame One. That means, the website makers could adapt their looping GIFs to start with the second half and play the first half afterwards. It will still loop, but as a static image, it would be a frame from the middle of the animation. So optimal would be frame One showing what's on the GIF. Doesn't work for longer movies in GIF format of course.