I was trying to play minecraft in the browser, and I got asked to install a plugin called "IcedTea-Web Plugin (using IcedTea-Web 1.2 (1.2-2ubuntu1))" (something related to Java 6), which I can see now in Tools > Addons > Plugins. There was another plugin to install (for Java 7), I didn't know if I had to select one, so I just clicked Install or Accept.