The first Beta build arrived today, and according to the official release notes it will enable support for H.264 playback on GNU/Linux operating system if the cross-platform FFmpeg multimedia backend is installed, as well as WebM/VP9 video playback on machines that don’t support MP4/H.264 content. Unfortunately, at the moment of writing this article, Mozilla didn’t write anything about the GTK3 integration for Linux, which failed to arrive in Firefox 42.0 (as initially promised), nor Firefox 43.0.