Sorry no, Private Browsing can't be enabled for one tab, or even one window. When you enter Private Browsing mode it is for all tabs and windows that are open, or that you open as long as you're in the Private Browsing mode.

The only way around that is to run a 2nd Profile in the Private Browsing mode, which will give you a window that is running in a separate firefox.exe process from the 1st Profile.

I'm a long time Firefox user but lately this is the feature I miss mostly. I think that would be the most important new user feature for Firefox 5 (which I'm using as a beta). I reported this as a new feature request.

In the meantime, if the only solution is to have 2 windows, it's really not important for me whether I run 2nd Firefox profile, or Opera which already supports this.

You can switch on Private Browsing temporarily and open a page in a tab and close that tab and leave PB mode before (re)opening links in other tabs as a workaround. You may see entries in Clear Recent History in this case, but there should be no cache entries or cookies left.

• You enter Private Browsing mode if you select: Tools > Options > Privacy > History: Firefox will: "Never Remember History"
• To see all History and Cookie settings, choose: Tools > Options > Privacy, choose the setting Firefox will: Use custom settings for history
• Select or Deselect: "Permanent Private Browsing mode"