At the moment, entering private browsing is a mostly synchronous operation, which depends on collecting the current state of the session. The ongoing work on bug 829904 is expected to make this collection asynchronous. We'll need to adapt private browsing accordingly.

Issues regarding global PB are moot in practice now that we have per-window PB in Firefox 20+. Specifically, the code that you're talking about no longer exists anywhere besides Beta and Release. -> WONTFIX.

(Please file a new bug if you see any performance differences in opening a new private window compared to opening a new regular window. Thanks!)

I am thinking of http://dxr.mozilla.org/mozilla-central/browser/components/privatebrowsing/src/nsPrivateBrowsingService.js.html#l554 is that code somehow dead?

(In reply to comment #3) > I am thinking of > http://dxr.mozilla.org/mozilla-central/browser/components/privatebrowsing/src/nsPrivateBrowsingService.js.html#l554 > is that code somehow dead? Yes.

In this case, which entry points (or source files?) should I read to find out how it works now?

With per-window private browsing, there is no need to save any session data before opening a new private window.

FWIW, the MOZ_PER_WINDOW_PRIVATE_BROWSING is the build time flag that we're using on trunk and Aurora right now.