The popular video hosting and streaming website YouTube uses two technologies to stream videos to its users. Either by making use of Adobe Flash, or HTML5 Video, with the latter option marked as experimental for some time now.
Provided that you have a HTML5 capable browser, you can join the HTML5 trial on YouTube to use this technology on the site. The core benefit here is that you do not have to install or use Adobe Flash Player at all.
To do so, load this page on YouTube http://www.youtube.com/html5 and click on the join button on it.
If you have not visited that page for a while, you may notice that Google has added three new bits of information to it. The page not only informs you if your browser supports HTML5 Video, H.264 and WebM VP8, but also if it supports Media Source Extensions.
If you open that page in Firefox, Safari or older browser versions, then you will notice that the browser's don't support it.
Media Source Extensions explained
A highly technical draft document is available here that provides you with additional information about Media Source Extensions.
If you are using the HTML5 video player on YouTube in Firefox, you may have noticed that you do not get to select 1080p or 480p as the video resolution no matter what you do.
You may have also noticed that you do get to select 1080p if you leave the HTML5 beta and return to Adobe Flash Player.
Google recently switched to adaptive streaming on YouTube for 480p and 1080p videos by using Media Source Extensions. Since Firefox does not support that feature yet, the option to switch to streams that make use of it becomes unavailable in the browser.
That's why you cannot select 1080p or 480p anymore if you are using Firefox and have joined the HTML5 video trial.
The only way around this for now is to use Adobe Flash to play videos on YouTube.
Mozilla is working on an implementation of Media Source Extensions, but it is not clear yet when it will become available. You can follow these two bugs on Bugzilla to monitor the progress made in this regard.