It seems almost everyone here still don't fully understand the situation.
I'll try to clarify.
Firefox (Abrowser) since recently can play some patent-encumbered formats.
This includes H.264 encoded videos. For example, it is able to play the MP4
videos of Youtube. Try it and you'll see it works.
Firefox plays patent encumbered formats through another software called
GStreamer, a multimedia framework. Many programs on Trisquel use this
framework, including Totem, which is the default player. GStreamer plays
many different audio and video formats, patent-encumbered and unencumbered
(free formats). In fact, GStreamer on Trisquel by default plays many
patent-encumbered formats, including MP3 files.
Since Firefox plays the patent-encumbered formats through another program
(GStreamer), Mozilla can't be sued for infringing those patents. GStreamer
are the one at risk.
This is why Firefox does play free formats without the need of GStreamer. It
supports the free formats internally, since there's no risk.
Firefox can also be made to support all other formats GStreamer provides,
including MP3 files. It's not any more difficult to support those other
patent-encumbered formats. Mozilla just choose not to. It's their policy.
