Mozilla announced that to improve security, the Firefox 14 will implement a "Click to Play" model for plug-ins that requires the user to opt-in to enable plug-ins on a per-site level. By disabling plug-ins by default, the Firefox developers aim to enhance security, lower memory usage and lower CPU usage. Full details over here.
With the new Firefox Click to Play feature, when a page you have opened has a plugin that wants to load content, it will wait for you to click to confirm that you are happy for it to load (i.e. to opt-in). For example, in YouTube, the first time you visit the site with the Click to Play browser, the space where the video would be is replaced by the plugin icon, which you click to start the Flash that loads the video player and streaming video.
There are various plugins for Firefox which have similar functionality; NoScript, Flashblock and ScriptNo stop various java and flash embedded files from running/loading automatically. The popularity of these plugins with tech savvy Firefox users probably flagged up to the Firefox developers that it is probably a good feature for the main program.