how do i download html5 videos in ubuntu?
Depends on the site. Firefox has context menus for saving html5 video, but they don't work if the site uses a customized player. For example, if you visit http://www.mozilla.com/en-US/firefox/video/, you will be able to right-click and select the "Save video" option.
On YouTube, that won't work. In this case, click the YouTube logo in the address bar, then click "More information" in the mini popup, then click the "Media" tab in the big popup dialog, then find the video and select it, then click "Save As". Change the extension to webm before saving. Alternatively, you can copy the address and load it on some download manager like DownThemAll or wget via terminal.
However, on YouTube is simpler to use Video Download Helper. If you want to download because of performance issues, see FlashVideoReplacer. It can replace WebM and Flash on youtube with a different plugin, like gecko-mediaplayer and also provide download links.
The Simplest Way:
Just right click the Video frame that is playing the video, and select "Copy Video URL" (depending on the browser you use) and paste the link in the address bar, and the browser should ask you to save the file.
More Preferred Way:
In case you're fond of downloading the video you see, whether on YouTube (Mostly based on Flash) or any other site (in case of HTML5 videos), you have two options,
In case of download manager for Ubuntu, I'll recommend DownThemAll add-on for Firefox (default web browser for Ubuntu.
And for adding "Download" button in YouTube, first you'll need to install Greasemonkey in the Firefox and then, use this script to add that "awesome" button in YouTube. And good thing is that the this Greasemonkey script is available for other web browsers as well.