Firefox (v3.5x) private browsing mode as "default"

[Ex Nihilo]

You won't be able to save passwords in private mode... but otherwise there are few drawbacks... So here's how...

Bring up "about:config"

Set "browser.privatebrowsing.autostart" to "true"

Restart FF, and you should be running in private mode (private browsing options will be grayed out under Tools)

[Jessie]

hello Pollo Loco, is there a reason for why someone would want to run firefox in private mode?

[travisN000]

Here is the official explanation (..and instructions):

http://support.mozilla.com/en-US/kb/private+browsing

Start Firefox in a private browsing session

There is a setting in the Edit > Preferences > Privacy panel / window which will automatically enable Private Browsing whenever you start Firefox.

   1. At the top of the Firefox window click Edit > Preferences...
   2. Select the Privacy panel.
   3. Set Firefox will: to Use custom settings for history.
   4. Click to select Automatically start Firefox in a private browsing session.
   5. Click OK to close the Options window. Click Close to close the Preferences window.