/usr/include/firefox/mozilla/plugins/ is in firefox-dev 11.0+build1-0ubuntu4.
This file is owned by root:root, with mode 0o755.
..
/usr/include/firefox/mozilla/plugins/AStream.h
/usr/include/firefox/mozilla/plugins/BrowserStreamChild.h
/usr/include/firefox/mozilla/plugins/BrowserStreamParent.h
/usr/include/firefox/mozilla/plugins/ChildAsyncCall.h
/usr/include/firefox/mozilla/plugins/ChildTimer.h
/usr/include/firefox/mozilla/plugins/NPEventAndroid.h
/usr/include/firefox/mozilla/plugins/NPEventOSX.h
/usr/include/firefox/mozilla/plugins/NPEventUnix.h
/usr/include/firefox/mozilla/plugins/NPEventWindows.h
/usr/include/firefox/mozilla/plugins/PluginIdentifierChild.h
/usr/include/firefox/mozilla/plugins/PluginIdentifierParent.h
/usr/include/firefox/mozilla/plugins/PluginInstanceChild.h
/usr/include/firefox/mozilla/plugins/PluginInstanceParent.h
/usr/include/firefox/mozilla/plugins/PluginMessageUtils.h
/usr/include/firefox/mozilla/plugins/PluginModuleChild.h
/usr/include/firefox/mozilla/plugins/PluginModuleParent.h
/usr/include/firefox/mozilla/plugins/PluginProcessChild.h
/usr/include/firefox/mozilla/plugins/PluginProcessParent.h
/usr/include/firefox/mozilla/plugins/PluginScriptableObjectChild.h
/usr/include/firefox/mozilla/plugins/PluginScriptableObjectParent.h
/usr/include/firefox/mozilla/plugins/PluginScriptableObjectUtils-inl.h
/usr/include/firefox/mozilla/plugins/PluginScriptableObjectUtils.h
/usr/include/firefox/mozilla/plugins/PluginStreamChild.h
/usr/include/firefox/mozilla/plugins/PluginStreamParent.h
/usr/include/firefox/mozilla/plugins/PluginUtilsOSX.h
/usr/include/firefox/mozilla/plugins/StreamNotifyChild.h
/usr/include/firefox/mozilla/plugins/StreamNotifyParent.h