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This article covers the protections you can get with Private Window, what stays on your device, and what it can't do without help.
Table of Contents
What’s included in Private Windows
Deletes your browsing data after you end the session
When you close all Private Windows, Firefox deletes the browsing history, cookies, and site data from that session. Everything else on your device stays the same. Firefox also signs you out of any sites you signed into in that session.
Block trackers that follow you across sites
In a Private Window, Enhanced Tracking Protection (ETP) blocks known trackers, the scripts companies use to follow you. It also includes Total Cookie ProtectionTotal Cookie Protection, which stops a tracker on one site from seeing what you did on another. Your activity in one place can't be connected to your activity anywhere else, so it never adds up into a profile of you. Total Cookie Protection is also available in Classic Windows.
Harder to recognize, even without cookies
Sites can try to identify you from the particular combination of your screen size, fonts, operating system and hardware. That combination is a fingerprint, and it works even after cookies are cleared.
In every Firefox window, we block companies whose business is fingerprinting, and we give sites slightly imprecise details about your device. In a Private Window, Firefox automatically hides more of what makes your device distinctive, such as…
- The fonts you have installed
- How your screen renders images
- How powerful your processor is, and
- How your device rounds off math calculations.
Alert you about malware and deceptive sites
Firefox automatically alerts you about malware and deceptive sites.
When you download a program, Firefox checks the site you got it from against a list of sites known to spread malware. If the site is on the list, Firefox blocks the file. If it’s not, Firefox asks Google's Safe Browsing service whether the program itself is known to be harmful, sending some details about the file. That's the only check that leaves your device, and it only happens for downloads.
Firefox also checks the sites you visit against Safe Browsing’s list of sites reported as deceptive or known to spread malware. These include phishing sites, which imitate a real business to get your passwords or card details. If a site is on the list, Firefox warns you before it loads, so you can go back. The list stays on your device, so this check doesn't send your browsing anywhere.
This protection is also available in Classic Windows.
Ask sites not to sell or share your personal data
In a Private Window, Firefox automatically asks every site you visit not to sell or share your personal data, a signal called Global Privacy Control. You won't get this in a Classic Window unless you've switched it on in about:preferences through these steps. Whether a site honors the request is up to them, though privacy laws in some US states make it a requirement.
What stays on your device
Firefox only removes data from the current browsing session when you close the last Private Window. Everything else on your device stays the same.
- Bookmarks and downloads - Bookmarks you save in a Private Window stay in your bookmark list and toolbar. Files you download stay on your computer, although they won't appear in Firefox's Downloads list.
- Address bar suggestions - The address bar can still suggest sites you've visited and bookmarks you've saved in Classic Windows. If you don't want to see these suggestions, you can turn them off in Settings.
What to know
Private Windows help keep your browsing private from others who use this device. They don't make you anonymous online or clear all of your data.
- Some activity in Private Windows may still be viewed by sites, search engines, internet providers, or your employer (if you’re on a work/school network or computer).
- A Private Window doesn't make you anonymous online.
- It doesn't hide your IP address or your location.
Add more protection (if you’d like!)
Private Windows cover what happens on this device. A few optional things sit outside it, and you can add protection for those in settings.
- Use Firefox’s built-in VPN to make your location harder to trace.
- Switch to Strict ETP for stronger protection in every window. In a Classic Window, it blocks trackers from loading at all. In a Private Window (which uses Standard ETP by default), Strict ETP it adds two things:
- Tracking tags removed from links (Query Parameter Stripping). You've probably noticed links with a long string of characters after the question mark. That's usually an ID tying the click back to you. Firefox strips it before the page loads.
- Protection from redirect tracking (Bounce Tracking Protection). Some trackers bounce you through an invisible middleman site for a fraction of a second to tag you on the way past. Firefox spots those and clears what they stored.
Frequently asked questions
When should I use a Private Window?
A Private Window is useful whenever you don't want this session left behind on the device, or connected to your usual browsing.
- On a shared computer, so your history, cookies and sign-ins don't stay after you close the window.
- To sign into a second account without signing out of your main one.
- To look something up you'd rather not see again in your history or address bar suggestions.
- To start fresh with a site, since each Private Window session begins with no cookies or site data from the last one.
Why do some websites still recognize me when I sign into a site?
Because signing in tells the site who you are. While Private Window stops sites working out your identity from data stored on your device, it can't stop you handing it over.
Once you sign in, that site knows it's you for the rest of the session, and anything you do while signed in is attached to your account. That's true for both a Private Window and a Classic one.
Why do I have to sign in again after closing a Private Window?
That's the protection working. Your sign-in is kept with the rest of that session's data, and Firefox discards all of it when you close the window. Nothing carries over to the next one.
While a Private Window clears your session once you close it, it doesn't hide you from the network. To make your location harder to trace, read the “Add more protection” section above.
Why am I still seeing ads in a Private Window?
Firefox blocks trackers, not ads. Those are different things: a tracker follows you between sites to build a profile, while an ad is just content on the page.
Ads can still appear for two reasons. Many don't need to know anything about you since they match the page you're reading rather than your history. And if you're signed into a site, that site can use what it already knows about your account to choose what to show you.
If you'd like ads blocked as well, you can add an ad blocker from Firefox Add-ons on desktop or Android.
Firefox is made by Mozilla, a non-profit. We don't sell your browsing data, whether it’s in a Private Window or a Classic one.
