If you are using Firefox 3.6 or above version, then you can enable html5 rendering by changing the configuration of Firefox.
Type about:config in address bar.
Type html5.enable in filter, and double click to set the value true.
Enjoy the HTML5!
To test the page, please create a page named html5.html, and paste the following code:
<!DOCTYPE html><html><head><meta http-equiv="content-type" content="text/html; charset=ISO-8859-1"><title>SVG in text/html</title></head><body><p>A green circle:<svg> <circle r="50" cx="50" cy="50" fill="green"></circle></svg></p></body></html>
Save the page and open it in Firefox, you should be able to see green circle!
More information about html5 is here.