add ffmpeg to firefox for mp4 video support (#434)
add ffmpeg to firefox for mp4 video support (#434)


ricardorsierra/dockerfiles,michael-k/dockerfiles-jfrazelle,johnsandiford/dockerfiles,michael-k/dockerfiles-jfrazelle,johnsandiford/dockerfiles,jessfraz/dockerfiles,jessfraz/dockerfiles,koep/dockerfiles,johnsandiford/dockerfiles,ricardorsierra/dockerfiles,ricardorsierra/dockerfiles,michael-k/dockerfiles-jfrazelle,jfrazelle/dockerfiles,jessfraz/dockerfiles,jessfraz/dockerfiles,koep/dockerfiles,johnsandiford/dockerfiles,ricardorsierra/dockerfiles,koep/dockerfiles,johnsandiford/dockerfiles,ricardorsierra/dockerfiles,koep/dockerfiles,michael-k/dockerfiles-jfrazelle,koep/dockerfiles,jessfraz/dockerfiles,michael-k/dockerfiles-jfrazelle,jfrazelle/dockerfiles