A private window is one of the few privacy tools almost everybody has used, and one of the few that almost everybody half-understands. The other half is not a secret: both Google and Mozilla spell it out on their own help pages.
What it actually erases
Google says that after an Incognito session ends, Chrome keeps neither site data nor a record of the sites visited. Two things survive on purpose: bookmarks you saved, and files you downloaded.
Mozilla says a Firefox private window deletes the session’s history, cookies and site data when the window closes, blocks known trackers from following you between sites, and hides some of what makes your device distinctive — fonts, rendering details, processor information.
Both sentences hang on the same condition: when it closes. A private window left open for a week has cleaned nothing yet.
What it does not hide
Google’s wording is blunt. The websites you visit, and the organisations that run your network — your school, your employer, your internet provider — may be able to observe what you do in Incognito.
Mozilla is equally direct: a private window does not make you anonymous. It does not hide your IP address or your location. It blocks trackers, not advertisements. And the moment you sign in to a site, that site knows exactly who you are for the rest of the session.
So when is it worth opening
When the risk is local: a shared computer at home or at work, a borrowed phone, a machine in a hotel lobby. Or when you want to see a page without your own cookies attached — a price, a form, a second account opened beside your first without signing out of it.
Not when the risk is the network. It will not hide your browsing from a corporate connection, it is not a substitute for a VPN, and it will not make a site forget you after you have typed your own name into it.
Photograph: hands on a laptop keyboard. Photograph by Simon Hattinga Verschure, CC0 via Wikimedia Commons.
Sources: the official Google Chrome and Mozilla Firefox help pages.
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