Disable Additional plugins are required in Firefox

Disabling popups from plugins should be enough to also get rid of the dialog. Enter about:config in the location bar, add (right-click and select New) the Integer setting privacy.popups.disable_from_plugins, then set the value to 2. Possible values for this setting are:
privacy.popups.disable_from_plugins
0 Allow popups from plugins
1 Limit popups to dom.popup_maximum
2 Block popups from plugins
3 Block popups from all plugins (even on whitelisted sites)