Mozilla didn't want to support H.264 because it's not an open standard, but due to the popularity of the codec the company is caving in, albeit with a twist because Mozilla will rely on the host computers available codecs so the company won't have to license H.264.
H.264 is already available on the Firefox mobile browser. H.264 encoding and decoding functionality is built into the chips powering all smartphones, so Mozilla just had to tap those resources to provide the support. Firefox for Android 17 Beta should be able to run H.264 content natively and Firefox OS also supports H.264. Again this is likely due to smartphone hardware.
For Windows XP users, Flash will be used to feed the data and render the frames. Data will come to the user as an HTML5 video stream, with Flash only performing decoding tasks. It will look and feel like HTML5 video for the user.