location of Firefox extensions in Mac OS?

In my computer (MacOS X 10.7.5, Firefox 16.0.2), they are installed in :

/Library/Internet Plug-Ins

Here is a list of files inside this directory :

AdobePDFViewer.plugin
DivXBrowserPlugin.plugin
Flash Player.plugin
Flip4Mac WMV Plugin.plugin
GarminGpsControl.plugin
JavaAppletPlugin.plugin -> //System/Library/Java/Support/Deploy.bundle/Contents/Resources/JavaPlugin2_NPAPI.plugin
Quartz Composer.webplugin
QuickTime Plugin.plugin
Scorch.plugin
Silverlight.plugin
flashplayer.xpt
googletalkbrowserplugin.plugin
iPhotoPhotocast.plugin
npdivx.xpt
npgtpo3dautoplugin.plugin
nsIQTScriptablePlugin.xpt