Understanding Private Browsing

Private Browsing was introduced in Firefox 3.5, giving users the option of browsing the web without keeping track of their history.  A recent Test Pilot study recorded, among other things, the time users activated Private Browsing, and the time they deactivated it.  Though what happens in Private Browsing stays in Private Private Browsing – that is, neither Firefox nor Test Pilot records anything during the period – we did learn a few things about the timing and duration of of Private Browsing mode sessions.

Here are a few simple insights we’ve gleaned from the data.

Note: Test Pilot is an opt-in service for Firefox 4 Beta users.  We would never record a user’s activity for any reason unless they explicitly signed up for this study.  Again, we do not record any information at all during a user’s Private Browsing session – only when private browsing was entered and exited. You can read over the Test Pilot Privacy Policy here.

Activation spikes at lunch.

Though people switch into Private Browsing mode throughout the day, there are a few periods where activation surges:

  1. Lunch: users likely switch into Private Browsing during their lunch breaks.  We see a major spike between 11am and 2pm.
  2. After School / Work: users appear to switch on Private Browsing just after they’ve returned from work or school, which is around 5pm.
  3. After Dinner: there is another substantial usage peak between nine and ten pm.
  4. Late Night: a minor spike exists an hour or two after midnight.

The 10-minute window is the most common.

Now that we know when users jump into Private Browsing, the next question involves finding out how long users stay in it.  The 25th percentile stays on for about 4 and a half minutes, the 75th percentile around 22 minutes, and the median stays in for about 10 minutes.

This trend appears to hold over the entire course of the day, with the notable exception of 5pm.  For some reason the median and lower quantiles are lower than the other hours.

We’ve more insights to glean from the Week-In-The-Life study.  Stay tuned for more.

20 responses

  1. Mike wrote on :

    Yeah, 10 minutes sounds about right. Anything longer than that and you’re probably doing it wrong.

  2. tom wrote on :

    i suspect the 5pm peak is checking something NSFW just before you leave work (and thus, it’s often just a short check because you are in a hurry to get out)

  3. Staś Małolepszy wrote on :

    I noticed that in the first chart, the data on the Y axis is in absolute numbers (“number of activations”). This begs a question 🙂 Has the data been normalized to account for the fact that people might generally use the browser more during the lunch hours?

  4. Ian Thomas (thelem) wrote on :

    Do you have information about whether the users are at home or work, or the total length of the firefox session? (you could assume that a ‘work’ session started between 8am and 10am, and a ‘home’ session started between 5pm and 8pm).

    I suspect you’d see quite different patterns of usage between the two scenarios.

  5. James Sullivan wrote on :

    I use it when testing a web app as a fresh user that has never seen the site – so, 10 minutes sounds like my average too.

  6. colinbashbash wrote on :

    SP: Quartile not Quantile.

    This trend appears to hold over the entire course of the day, with the notable exception of 5pm. For some reason the median and lower *quantiles* are lower than the other hours.

  7. colinbashbash wrote on :


  8. Hamilton Ulmer wrote on :


    This is a good point. We should have addressed this. Generally, there is not a spike in usage at lunch for other statistics of interest. For many other metrics of interest we see a familiar sinusoidal wave of usage, with the low point at around 3 or 4am, and a high point at approximately 6-7pm. So spikes like the one at noon are pretty interesting for that reason alone. Good catch, though 🙂

  9. Peter Tanham wrote on :

    My first thoughts were similar to Staś – would this not tell us more if it showed private browsing sessions as a % of total browsing sessions, over the course of the day?

  10. Ken Saunders wrote on :

    It’s really unfair and inaccurate for people (like this one http://thenextweb.com/apps/2010/08/23/firefox-uncovers-most-common-10-minute-periods-of-private-browsing/) to assume or imply that people are doing naughty things in Private Browsing mode based on the amount of time that one spends using Private Browsing mode.

    Perhaps they’re doing online banking or shopping.

    Besides, it takes much longer than 10 minutes to find decent porn?
    Or so I’ve heard. 😐

  11. Alex Faaborg wrote on :

    >Perhaps they’re doing online banking or shopping.

    Stanford’s security research group looked into the actual content that people are viewing in private browsing mode:


    They then went as far as to criticize browser vendors for saying that it was for buying surprise presents, conclusively exposing that as mostly a euphemism.

    Overall an interesting study, although I disagree with their assumption that our family friendly use cases were at any point meant to be perfectly representative of actual usage (although it’s admittedly hard to detect sarcasm in scientific paper format).

  12. Jeff K. wrote on :

    Besides, it takes much longer than 10 minutes to find decent porn?

    Allow me to repurpose a quote from upthread:

    [Y]ou’re probably doing it wrong.

  13. Jeff Flowers wrote on :

    Screw Stanford. I don’t see why it matters what people are using Private Mode for. Personally, I use Private Mode all of the time and I have had to resort to using NoScript because web developers of major sites are jerks.

  14. Eric Adam wrote on :

    My Grandaddy always said, “I don’t care what people do, as long as they don’t do it in the streets and spook the horses”.

  15. Ken Saunders wrote on :

    @Jeff K

    Perhaps I should have written,
    “Besides, it takes much longer than 10 minutes to find decent porn that one has not already seen”

  16. Keith Wansbrough wrote on :

    The lunchtime spike suggests a misunderstanding of what Private Browsing achieves. If it’s a work computer, your employer can still tell what you’re browsing (if HTTPS, at least which site you’re looking at, otherwise everything). Does FF need to do more education here?

  17. dsfsdfsdfs wrote on :

    One question. Why Firefox closes current session when starting private browsing, unlike Opera, Chrome and IE? Will you implement this in new versions?


  18. Jack wrote on :

    How is private browsing private if absolutely everyone can access that browsing data, including the start/end times and the actual sites visited? Amazing.

  19. James wrote on :

    Possibly a naive question: Is there any kind of geographical filter on the data?

    12pm in the US is approx 5pm (here) in the UK, and plenty of people would i’m sure log in for 5-10 minutes for NSWP before heading out, assuming similar usage patterns across timezones/regions might you not just be getting a spike in the results as a cumulative result of confluences between ‘key’ times in the US vs. other ‘key’ times elsewhere?

  20. Tahir Jamil wrote on :

    Private browsing is need of time. A lot of hackers and community stalkers are tracking internet surfing and IP history. There should be way that people surfing or IP history cannot be traced by any means so that they could surf in peace of mind. A must need of future browsers.