This add-on is deprecated and no longer supported. Starting with Firefox version 3.6, this feature was rolled into the core so this add-on is no longer necessary.
There seems to be a lot of confusion as to what this extension is and what this isn't.
This add on, very simply, allows you to enable private browsing from the command line by adding the "-private" flag. Having an additional command line parameter is beneficial for anyone who wants to launch Firefox directly in private browsing mode either from a shortcut, a launcher, or the command line.
With this extension installed, simply append "-private" to the Firefox path without the quotes in the shortcut and any time you click it, Firefox will start in private browsing mode. See the screen shot to see how this is accomplished in Windows.