I'm working with Firefox 43.0.1 on Windows 7 Pro 64 bit. I just noticed an odd behavior, and I don't think Firefox used to behave this way.

I had a GMail tab open in private browsing mode, and then opened a new tab in that same window to go to YouTube. YouTube recognized who I was and asked which account I wanted to use to browse YouTube with. I promptly closed that tab, opened a new private browsing window and went to YouTube and it auto-logged me in as one of my accounts.

I was surprised by this behavior, though I'm not sure that I have previously tested it. I expected that private mode would not allow webpages/sites access to other cookies and such, for purposes of limiting tracking and whatnot. Even if it does not do this per tab, I expected each window to be pseudo-seperate. Therefore, I would expect the above scenario to not occur and YouTube to not know anything about me and not auto log me in.

How is Firefox supposed to manage tabs and windows in private browsing mode? Is Firefox supposed to allow websites access to the sort of data that knows if I am logged in or not? Are my expectations realistic? Am I expecting too much?

I found this Mozilla Wiki page explaining exactly what I expected, and if I am reading this right, it says that it was implemented circa Firefox 20.

I am also running several other privacy and security minded extensions, so maybe I previously experienced what i expected due to the combined effects of Firefox and the extensions.

  • I've noticed this as well with my boot camp windows 7 pro 64 bit. I had it set where it would clear cookies everytime I closed the window. And sure enough if I reopen soon after closing in private browser I'll be auto logged in to Google accounts. I'm interested in seeing an answer, and perhaps it is a Google thing.
    – RB4
    Commented Feb 10, 2016 at 3:18
  • Private Sessions dont interact with the data of your normal sessions. But normals sessions can access the data of other normal sessions (i. e. Youtube can access the Google Coockie). Private browsing behaves the dame way. Private and normal browsing cannot interact with each other. Private windows can access cookies and such set by other private windows. This will reset when you close all private sessions
    – BlueWizard
    Commented Feb 11, 2016 at 5:40
  • Just a quick note: Using Chrone Incognito, Firefox private tab or variours doesn't make you anonymous. It's not even close to anonymization. It's just a regular tab that will expose every data that it would send otherwise too. The only difference is the rollback it performs when you close the private window. The private session wont dtore any data on your device but it still exposes various data to the web provider and wont make you more secure or whatever on that end
    – BlueWizard
    Commented Feb 11, 2016 at 5:43
  • If this behaviout of firefox bugs you really hard you can use plugins ahich allow you to have multiple independent normal/private windows which wont interact with eachother
    – BlueWizard
    Commented Feb 11, 2016 at 5:44

1 Answer 1


Private Browsing and you

I had a GMail tab open in private browsing mode, and then opened a new tab in that same window to go to YouTube. YouTube recognized who I was and asked which account I wanted to use to browse YouTube with.

Private browsing in Firefox is instance-based. This essentially means if you log into any google services, you will be detected during that session. If you have a single private browsing session, and you log into a website, close the tab and then go back later, you will still be logged in because you haven't terminated that private browsing session.

By default in Firefox, private browsing is not per-tab. It's per instance. For that particular session/instance, you should expect everything to operate like a normal browser, unless you disable cookies/etc.

YouTube belongs to Google. You'd use your Google account for YouTube, etc. In that particular private-browsing session, it is detecting your Google account since, as I've said before, private browsing does not create a new instance of itself every time you open a new tab.

Browser Fingerprinting

Keep in mind that changing your IP and going incognito will not protect your identity from browser-fingerprinting, among other things.

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