But for some reason, Firefox refuses to play back those kinds of files.
And that reason is because x264 is a free and open source implementation of the H.264 codec, and you still need to pay a license to use the patented technology regardless of how you do that. Using a free implementation of the code doesn't get you a free license for the codec.
Or, if I write a short description "A couple walks hand-in-hand through a park at sunset. The wind rustles the orange leaves.", I don't think it would be surprising to anyone that an image or video of this would be relatively huge.