Firefox Private Browsing Mode Bypassed By Add-Ons

Martin Brinkmann
Feb 24, 2010
Updated • Jun 20, 2016
Firefox, Firefox add-ons

One of the greatest strengths of the Firefox web browser is the Mozilla development community that has produced thousands of add-ons to improve the browser or the browsing experience in the past.

Update: Mozilla fixed the issue in the meantime.

Mozilla introduced private browsing recently in Firefox. Private browsing mode prevents the permanent storage of the browsing history, cookies and other information.

While Firefox itself prevents local data from being remembered after private browsing sessions, add-ons until now have not necessarily as there was no requirement for add-ons to respect the private browsing mode of the browser.

firefox private browsing

This means that data may still be saved on the local system while in private browsing mode if add-ons are installed that ignore the mode.

That's obviously not the intention of private browsing mode, and Mozilla has addressed the issue in a new blog post linked here.

Mozilla explains that private browsing mode has been designed to prevent that browsing data such as the browsing history is stored on the local computer. Browsing data refers to data that can be used to identify the websites the user was on. This includes website addresses, cookies or temporary data.

Mozilla outlines how it wants to go forward with the mode in regards to browser add-ons:

  • If your add-on stores browsing data in any way, it must support PBM. This support cannot be disabled in any way, not even with hidden preferences.
  • If your add-on stores some other type of personal data, support for PBM is optional. What we did with is a good guideline: have a preference “support PBM for the data this add-on handles”, turned on by default.

Mozilla will enforce this new rule starting at the end of March 2010. Add-ons that are updated or newly added to the Firefox add-on repository after that date will be rejected if they do not comply with the new privacy rules. The rules will however have no effect on add-ons already listed on the extension gallery.

This means, at least in theory, that you may run add-ons right now that are not in compliance with the new rule set. Also, after March, you may still download add-ons that are not in compliance if they have not been updated in the meantime.

This is for instance the case for old add-ons that have not been updated in a while.

Firefox Private Browsing Mode ByPassed By Add-Ons
Article Name
Firefox Private Browsing Mode ByPassed By Add-Ons
Firefox's private browsing mode may be bypassed by add-ons for the web browser. Find out what Mozilla has done to prevent this.
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  1. Jenn said on March 15, 2011 at 12:16 pm

    The reason why people used this add-on was so you FF doesn’t close all your opened windows and tabs when you choose ‘Private Browsing’. Both IE & Google Chrome don’t close all your work the way FF does.

    Why doesn’t FF just update their user interface so Private Browsing sessions don’t close all your windows anymore?

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