Firefox users without H.264 platform codecs were affected: 100% of Windows XP users (which are about 14% of all Firefox users worldwide!), 2% of all other Windows users (the "N" and "KN" editions in Europe and South Korea), and 3% of Linux users.
OpenH264 can't be used for video decoding because it can't decode the H264 profiles used to encode video on the internet; it's strictly baseline profile only which is pretty useless. The only reason it exists is because Cisco has a bunch of H264-only videoconferencing hardware that they want to be able to work with WebRTC.