Google has been offering a so called HTML5 trial on the video hosting site YouTube for some time now. Visitors of the site can join the trial to use HTML5's video capabilities instead of those made available by Adobe's Flash plugin. While that is great as it reduces the dependency on third party plugins, it may at the same time be limited to select videos depending on the browser that is being used.
The main reason here is missing support for one of the standards used by YouTube to make videos available. Mozilla Firefox for instance supports WebM but not h.264. While that is going to change soon, it does not seem to have an impact on the availability of videos on the site.
Firefox 22, currently available as a Nightly version, supports both h.264 and WebM, and even if you join the HTML5 Trial on YouTube, you will notice that some videos still won't play. The error message displayed on the site refers to a missing installation of Adobe Flash player. I can't say if YouTube is using browser identification to block the playback of videos, or if there is another reason for that right now.
I'd like to share with you an option to play all videos on YouTube without Adobe Flash plugin even if h.264 is not supported by the web browser.
All you need to do is point your browser to the following address: http://m.youtube.com/
This is YouTube's mobile subdomain, and when you open it and the videos that are displayed on it, you will notice that they all play fine on the site even if you do not have Flash installed on your system.
I have only tested this in the Firefox web browser - both Firefox 19 and 22 - and it worked in both after I joined the HTML5 trial on the site.
You can obviously try to play a video on the main version of the website first and if that is not working, you can switch to mobile by replacing the www. part of the address with m. to do so.
Please note that you need to actively switch to the desktop again as you can't replace the url the other way round. The easiest way to do so is to load the following url in the browser: http://www.youtube.com/?nomobile=1