A SECURITY FEATURE which gave punters total privacy has been dumped from the final version of Firefox 3.
Private Browsing would have disabled all caching, cookie downloads, history records, and form data during the session.
If it worked it would have meant you could surf the Web and leave nothing sticky on your computer.
Mozzarella Fountain's big cheese in security Johnathan Nightingale, said that Private Browsing was, in principle, pretty cool. It would mean that what you were about to do would not be logged anywhere.
You hit a button and everything past that point isn't logged. Then you hit the button again and you were visible again.
It would be handy while you were borrowing a computer and didn’t want your mates to see you had been checking out porn, violence, or Apple's product pages.
Nightingale said, however, that the main problem with the button is that it touched a lot of code. It was likely to interact with Web sites and mashups and things like that and was just a crash waiting to happen. µ
L'Inq
News.com
I guess we'll have to stick with manually wiping all our cookies, history etc.

I honestly can't see this being a feature I'd use that much, if at all. It sounds like feature-bloat (and suspect it'd make FF more resource hungry).
features being shed to make release dates...
I think it more likely that either some US security agency has had a quiet word and told them it doesn't like features that may prevent it looking through confiscated hard drives for evidence, or Mozzarella's PR people are worried that it might become the browser of choice for the world's ne'er-do-wells and this wouldn't be a nice angle for the press to approach future versions...
www.diskcleaner.nl
That's what they are there for. Install the add in "Stealther" and this will do exactly what's needed.
Opera has been doing this for years, under a nice menu item called clear privacy data. Cookies, addresses, cache, transfers etc etc are all deleted with two mouse clicks.

Its a simple enough idea, I wonder why they can't code it?
It's an unfortunate loss, but the feature is surprisingly easy to replicate if you put a little elbow grease into it. These instructions are Windows-specific but can be adapted to any platform.

1.) Start Firefox with the Profile Manager:
firefox -ProfileManager

2.) Create a new profile, call it "secure."
3.) Create a shortcut/link to Firefox that uses this profile:
firefox -no-remote -P secure

4.) Customize the profile to turn off cookies and caching, history, javascript, and whatever else. Also tell it to clear all privacy data on exit, just to be safe. You can use a tool like foxmarks to share select bookmarks.

Firefox keeps this profile data separate, so the "regular" browser will keep your history data but the secure one will not. This gives you options when browsing. Hope this helps.
Keeping multiple profiles around is easier with an extension such as profileswitcher (https://nic-nac-project.de/~kaosmos/profileswitcher-en.html) .

I use this tool on a daily basis.

I haven't used SecureBrowse (https://addons.mozilla.org/en-US/firefox/addon/5967), but it claims to do the clean-up mentioned above by others.

Also, you can configure your browser to clean our your data when you exit quite easily.

Tools -> Options -> Privacy on Windows
Edit -> Preferences -> Privacy on Linux
There's an extension called Distrust that does pretty much this exact thing...
Safari has it, "private browsing".
You can just put ~/.mozilla on a ramdisk
Like Opera, you can clear cookies, cache, history, etc. very simply in Firefox by using Clear Private Data in the Tools menu. You can even ask for private data to be cleared whenever you exit the browser. Presumably the difference here is that "Private Browsing" would have prevented the information being stored in the first place rather than simply deleting it afterwards.
Try Browzar. Does the same. Only its too simple and aint have no plugins and stuff...I think. But does the "wipe all after use" thing too.