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.
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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