The Mozilla developers have announced that Firefox 43.0, scheduled for release on December 15 2015, will be replacing GStreamer with FFmpeg. This is not surprising at all, since FFmpeg is available on the most popular Linux systems by default, or make it available via the default repos.
Also worth mentioning, Ubuntu 15.10 has also adopted FFmpeg, replacing Libav, which is a FFmpeg-fork. For now, FFmpeg PDM has been implemented on Firefox 43 beta only, but it will reach the stable version also.
If you are new to this, FFmpeg is a set of multimedia codecs that permits the users to play and manipulate a lot of file formats. And because it is cross-platform, it can be installer or compiled on all the existing operating systems, including the ones running on X86, ARM and PowerPC architectures.