H.264 video support lands in Firefox Nightly. Update: Not
Did Google play Mozilla in regards to support for HTML5 video standards? Back when HTML5 video began to take forms, browser developers split into two groups. There were Microsoft and Apple who supported the widespread H.264 video standard, and on the other side Mozilla and Opera supporting the WebM video standard instead. Google, which more or less owns the WebM standard, pledged to support it as well but also supported H.264 in its browser at that time, but with the promise to change that.
The problem for Mozilla and Opera was that Google did not keep its promise until now, which meant that the companies supported the WebM service which even Google's own video hosting portal YouTube did not use for all of the videos offered on the site. Google Chrome users did not experience any issues as Chrome was still supporting H.264 so that all videos on YouTube played just fine even with Flash disabled in the browser.
Firefox and Opera users on the other hand ran into video playback issues on YouTube and other video sites due to missing support for H.264.
Mozilla a month ago announced that it made the decision to no longer wait for Google to come through. Engineers of the company began to implement changes to the browser to bring H.264 support to Firefox after all. The engineers decided to use H.264 if the host operating system made them available on the computer. Firefox Mobile was the first version of the browser to receive H.264 support.
If you are running Firefox Nightly on Windows, you may also notice that H.264 is now officially available in the browser. You can test that for instance on YouTube's HTML5 page which lists the browser compatibility with the standards.
What this means is that Firefox users can now play back all videos on YouTube and other HTML5 video sites. YouTube videos play even if Adobe Flash is not installed. It may take some time before H.264 support is added to the stable version of the browser. It is likely going to happen in one of the next updates. Note that H.264 is available natively on Windows Vista and newer only, which leaves Windows XP users with no other option than to keep on using Flash to play videos.
Update: It appears that H.264 support has not landed yet in the Nightly version of Firefox. On HTML5test for instance, support for H.264 is not detected. I'm not sure why YouTube is showing that the browser is supporting h.264 though.
Update 2: Found the culprit. NoScript seems to have blocked the detection script on YouTube. With NoScript blocking the YouTube domain, h.264 is shown as green and thus supported by the browser, with NoScript allowing scripts to run on youtube.com, h.264 turns to red, but the other two stay green.Advertisement