Privacy Plus adds a small but important feature to your browser that provides you with better privacy control.
Web sites commonly use Flash LSO cookies (Local Shared Objects) to track you between sessions.
Cleaning the browser History does not delete these cookies and your session is being tracked even across different browsers.
Yes, you read that right, even when you switch to another browser for certain activity and even when you start Firefox's or any other browser's Private Browsing feature, the traces follow you back to your regular session in Firefox.
That backdoor provides web site a way to workaround basic cookie and history cleaning.
But that's not all. Your privacy is compromised even with other people whom you share the computer with.
By simply going to Adobe Storage control panel, anyone can see which website you were visiting (considering these website are using LSO) You can check it easily yourself by going to:http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager07.html
What can you do?
All you need is to delete the Flash LSO files whenever you clean the regular standard web history.
Privacy Plus adds a simple checkbox (Clear Flash Local Shared Objects) to the [Clean Recent History] dialog.
(The panel is accessible from Menu->Tools->Clean Recent History)
You can open the panel, and delete those cookies.
Why Firefox does not provide this feature built-in?
Probably because Flash in general and LSO in particular are not part of the Web Standards.
When you installed Flash (and any other plugin for that matter) you revoked the browser from interfering with its methods.
Does cleaning LSO is dangerous?
However, keep in mind that as deleting the regular History makes your browser to "forget" anything you done,
the same will happen when you'll delete Flash LSO.
Login credentials and various other local web site settings will be purged.