From MozillaZine Knowledge Base

Up to Plugins

To completely disable the yellow plugin bar that tells you that you're missing plugins, use about:config to set the appropriate preference:

• Firefox 3 and above: Set plugins.hide_infobar_for_missing_plugin to true.
• Firefox 2 and below: Set plugin.default_plugin_disabled to false.

There is currently no way to disable the bar for only certain plugins or for certain sites.

External links

• Bug 261655 - Can't completely disable the new "Install Missing Plugins" infobar
• Bug 268936 - allow user to disable "additional plugins required bar" for given plugin