I am have not upgraded to Firefox 4 yet because I am waiting for the bugs to shake out. The Register reports a potentially serious issue.
… On Tuesday, members of the US-CERT echoed the advice that WebGL be disabled. So far, neither Context nor CERT has given instructions for turning off the standard in various browsers.
In Firefox 4, type “about:config” (minus the quotes) into the address bar and set webgl.disabled to true. In Chrome, get to the command line of your operating system and add the –disable-webgl flag to the Chrome command. On a Windows machine, the command line would be “chrome.exe –disable-webgl”.