I think gimp can make GIF animations, but I don't think it will import video files. However you can still save the frames you want into image files and then use the GIMP.

That's a good idea. Just exporting a few frames might be even better for what I want then a full gif video.

Thanks for the suggestion!

you don't. you use MNG (motion network graphics, related to PNG), which is a free format without potential legal problems.

apart from that, no idea.

But MNG is not supported by most browsers as far as I know. That includes Firefox.

And gif patent has expired.

MNG development sort of stopped.