Hello! Long time no see!
It’s been a busy period and I have lots of news to share. First of all, I decided to have a look on the Mozilla Add-on SDK . It has a very simple API to create Add-ons for Firefox.
Anyway, I tried to come up with an idea of what would be my first Add-on. Hmmm…An Add-on that can make my web experience less annoying. Considering that I spend half of my time on YouTube to listen to songs (mostly), view videos etc, as a Linux user, I get really annoyed when the Flash plug-in crashes and I have to restart Firefox. You can always visit youtube.com/html5 to change that but what if you delete your cookies? It’s a boring procedure.
So, what I thought was to make an Add-on that would simply add the “html5=1” parameter on the URL. And I did it…well, kind of, it’s now an experimental Add-on for Firefox. I need to add some more features for it to be considered as a proper Add-on. It’s called “YouTube HTML5 Switch” and here it is at the Mozilla Add-ons website, and here is the source code on Github.
I currently develop the Add-on at the Add-on Builder (that means online). I will eventually download the SDK and try it on Fedora 🙂 It’s not the smartest Add-on in the world, but I think it’s a good start for a newbie. By the way I need to say that the SDK’s documentation is not very helpful and I needed to google a lot to write down a few lines of code. Anyway, in every “major” release I will be posting here any changes etc. You can also read the README.md on Github.
What’s more? Wonky Doll and the Echo (the band where I play) are supporting I Like Trains here in Athens on December 15, 2012. You can check our Bandcamp page and listen to our songs. Now, if you have installed the Add-on you can test it with these video…if you go on YouTube of course 🙂
Some videos like this for instance don’t have an HTML5 player so the plugin will not be of any use here.
Other videos though, do have have an HTML5 version and the plugin will work!
Kostas Antonakoglou 
It’s funny that you say that that video doesn’t support HTML5… yet there it is, in HTML5. 🙂
This is because all Youtube videos DO have HTML5+webm, it’s only in the embedded version.
I’m not sure if it’d be possible, because I’m not much of a programmer, but couldn’t you potentially write a userscript or add-on that does some sort of re-embed of the embedded player, onto the youtube page?
Not all videos on YouTube have been converted to WebM. I haven’t really made any research on this matter but this is what makes what you proposed difficult to be done. I thought about it too.
This Add-on is my first attempt to write something with the Add-on SDK, so I wanted something easy! It can’t get any easier than that haha 🙂