Plugins are add-on programs that allow you to view non-HTML content such as PDF files, Flash content, Java applets, and video within Firefox browser windows, or in separate windows created from Firefox windows.
If a plugin is missing when you view a page that carries plugin content, then you’ll see a jigsaw piece in place of the content; a yellow plugin information bar will also appear at the top of the page for eg if Macromedia Flash plugin is missing.

To install the required plugin automatically, just click the Install Missing Plugins… button and let Firefox find the plugin for you. Follow the resulting install prompts as you normally would. At the end of that process, the current page is redisplayed with the plugin content presented by the new plugin.
Major plugins are available for Adobe Acrobat, Macromedia Flash, Java, Apple Quicktime, Realplayer, and Windows Media Player. Firefox knows how to get all of these plugins. However, if, by some chance, the plugin isn’t found by Firefox, or you want a particular version of the plugin or some other special arrangement, you can install plugins yourself. You can obtain most plugins at https://addons.update.mozilla.org/plugins/.
To install plugin software from that page, look for the link that suits your operating system, click it, and follow the instructions on the resulting page. Supporting documentation is also found at the PluginDoc project. You can refer to that more extensive site should you have any problems