Starting with Firefox 26, plug-ins require user interaction to be activated. With the exception of the popular Adobe Flash Player, all plugin content embedded in a Web page is now disabled by default and enabled by a user’s click. This change has been made in order to improve security and performance of the browser. See the Mozilla Security Blog and the Future Releases Blog for details. Web developers are encouraged to leverage modern Web technologies, including HTML5, to provide richer user experience. If that is not possible, see the site author guide for click-to-activate plug-ins.
Update: Mozilla has decided to hold off turning on click-to-activate for all plug-ins except Java, on the Beta and Release channels, until a whitelisting strategy is defined.