To the people who say "THIS ADDON IS NOT NEEDED ANYMORE," you don't know what you're talking about!
When you enable PB, all your open tabs and windows will be CLOSED, and FF will open a new PB window. You'll have no access to any of those closed tabs and windows while in PB mode.
When you disable PB, all your old tabs must be reloaded. This can be very time consuming and frustrating if you're on a slow or poor connection like I am now. For my 3 windows and 61 tabs to reload can take more than 2 minutes, and I may have to reload them several times and turn off/on Airport a few times because it loses connection with the router.
When you use Stealther, you can enable it on the fly, and all your other windows and tabs remain open. This allows you to quickly check out a page or site without leaving tracks, then you click the button again and you're back to normal browsing - no need to reload all your tabs again.
Unfortunately, as other reviewers have noted, it does NOT seem to work on FF 3.6. I've installed it twice, and the button doesn't appear in the Menu Bar's Customize menu, nor does the Preferences button work in the Add-ons window.
The developer should make the distinction between Stealther and Private Browsing more clear in the description, so people won't be confused. But first he need to make it work with the current FF versions - I'm using 3.6.10.