Yeah I'm struggling to determine if this really is a big step. I'm not sure what the difference is between "Firefox Nightly" and regular old Firefox. But honestly, most browsers support H.264/MPEG-4 AVC at this point(especially mobile). I would really like to see this become the primary codec for streaming web video. It has excellent compression ratios, much better than webm or flv and it's the perfect encoding type for HD videos.
Nightly is the build generated each night from the latest trunk code. After 6 weeks, nightly becomes aurora, which is more stable (not updated each night). Then after 6 weeks, it becomes beta and is tested by a wide audience. After 6 more weeks, it is released as the current stable version.
It's big for anyone doing video because it means you no longer need to encode everything in two formats simply to support Firefox.
Beyond saving half of your disk space this is nice because WebM has limited toolchain support so anyone with a video production pipeline basically had to decide whether it was worth the effort integrating a bunch of new tools or continuing to put up with Flash.