Firefox: Lightweight Themes not working in Private Browsing Mode right now

Martin Brinkmann
Apr 18, 2013
Updated • Apr 18, 2013

Mozilla implemented a much requested feature recently in Firefox that enabled per-window private browsing in the browser. Previously, Firefox users were only allowed to run the browser - and all of its windows and tabs - in private browsing mode or in regular mode, but there was no option to mix the two using a single user profile. The change enables you to run regular Firefox windows next to private browsing windows.

The API for that feature is actually more powerful than that, as shown by the per-tab private browsing add-on for Firefox that allows you to run individual tabs in a regular browser window in private-browsing mode.

Anyway, if you are using lightweight themes in Firefox and the private browsing mode, you may have noticed that the themes are no longer working when you open a new private browsing window in the Firefox browser.  While they work just fine in the regular version, you will notice that Firefox reverts to the default theme as soon as you launch a new private browsing session. The theme does not get uninstalled or anything, it is only blocked from this mode.

There is nothing right now that can be done about it. Mozilla is aware of the bug and it is likely that it will be re-added in Firefox 22 21. While there are not any verifications for this, it appears that the reason for the change is the way private browsing windows are themed.

Basically, we theme the private windows the same way that we apply a persona to a window, which means that we cannot use the persona infrastructure to private windows since they already have a "persona" applied to them (or at least a persona of some sort.)

Others have suggested that this has been done to make private browsing mode more visible to the user. This is not the only indicator though on the screen. Firefox displays a clearly visible purple Firefox button and icon in private browsing windows as well.

firefox private browsing

Anyway, lightweight theme support in Firefox's private browsing mode will be enabled again in a coming release. Note that only lightweight themes (formerly known as Personas) are affected by the change, while complete themes are working identical in both browsing modes.


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  1. baju import said on August 29, 2014 at 3:47 am

    Okay here are the details. I want to add a new button to firefox, the purpose of that button is that when i type something in the address bar in firefox and click that button in navigation toolbar, that must serve the purpose of “I am feeling lucky button on google homepage”. Any super genius here?.

  2. Caspy7 said on April 18, 2013 at 9:47 pm

    Originally they were going to have a modified, darker theme (in the same manner as Chrome) to give a more stark indicator of Private Browsing windows, but there were issues with this (not technical ones if I recall) so they reverted to the default theme. So the current state is the undealt with aftermath I suppose. They actually had to add the ability in the first place to allow two lightweight themes displaying simultaneously.

  3. Sören Hentzschel said on April 18, 2013 at 4:54 am

    Hi Martin,

    “Mozilla is aware of the bug and it is likely that it will be re-added in Firefox 22.”

    status-firefox21: — → fixed


    1. Martin Brinkmann said on April 18, 2013 at 5:24 am

      Thanks, corrected.

  4. Karl J. Gephart said on April 18, 2013 at 2:16 am

    I’m not surprised this is happening with Firefox. I’ve used the FXChrome theme forever and, when I used to use the addon called Private Browsing Windows, I would get no theme appearance whatsoever in the separate browsing window. Now that I use the addon, Private Tab, everything works seamlessly. It’s really the way to go for now!

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