However, manipulation of the browser window's interface and functionality will be extremely limited by definition, and even if it wasn't, the implementation of such abilities is nearly impossible to achieve in Web Extensions.
If you have installed the Flash plugin but it does not appear in the Add-ons Manager, open about:config and make sure that the preference has not been set to "false" (if it has, right-click on it and select Reset to set it back to "true").
 Note: If the Flash plugin is installed, make sure it is enabled in the Firefox (3.0 and above) or Sea Monkey 2 Add-ons Manager (see above); otherwise, the Flash test will not work and the "Shockwave Flash" plugin entry will not be displayed in the about:plugins list.
They are the the NPAPI test plugin, Adobe Flash, Apple Quick Time (Windows) and Microsoft Silverlight (Windows).
These plugins are specified in a separate plugins.enabled. We can disable them by changing their value to false. By default the preference plugins.enabled is already set to “false”. The plugins.timeout Secs is also not important here as other values are false.
Some websites require the use of the Windows Media Player (WMP) plugin for certain content such as embedded audio and video.