Hey there!

I've found this link to a case similar to yours: https://support.mozilla.org/en-US/questions/1212268

Hope it helps!

Bithiah

A little bit digging gave me that the setting "extensions.allowPrivateBrowsingByDefault" is indeed removed. This is sad, so for now there is no easy way to enable all addons in private windows at once.

Hi Tim,

• Click the menu button Fx89menuButton, click Add-ons and Themes and select Extensions.
• Click on the extension that you would like to manage.
• This will open a panel where you can manage extension settings.
• Next to Run in Private Windows, select Allow to enable the extension to work in private browsing,
• or select Don’t Allow to disable it in private browsing. Once you have made your selection, you can exit the Add-ons Manager.

See More: https://support.mozilla.org/en-US/kb/extensions-private-browsing

Note that there used to be a pref to allow extensions automatically in PB mode (extensions.allowPrivateBrowsingByDefault), but support for this pref has been removed in Firefox 91 as it was causing issues with extensions that weren't allowed to run in PB mode via their manifest.

I wrote a script to solve my problem - github repo. It updates extensions prefs in user's profile dir to allow extensions in private mode.

See also:

• 1627939 - Problem with extensions with incognito: not_allowed in their manifest and flag allowPrivateBrowsingByDefault