If Firefox is repetitively annoying you to install missing plugins with an infobar on top, you can easily disable it. If Firefox founds you’ve not installed plugins that required content to be displayed on sites, then it displays a message on top of page as “Additional plugins are required to display all media on this page and prompts you to “install missing plugins”.
You already knew you’ve not installed Flash or Java to speed up the loading of pages and to avoid vulnerabilities in browser, and the browser is warning you to install missing plugins is meaning less IMO, here is how to disable this warning.
1. Type about:config in the browser and then press enter.
2. Find the preference plugins.hide_infobar_for_missing_plugin and double click on it to turn its value to “True”.
That’s it! From now onwards that infobar will not appear when you load a page that requires plugin.