Themes can always run in private windows, so changing the pref doesn't do anything. The private browsing settings should be hidden for extension-based themes.

(In reply to Mark Striemer [:mstriemer] from comment #0)

Themes can always run in private windows, so changing the pref doesn't do anything. The private browsing settings should be hidden for extension-based themes.

I just briefly tried locally and the private browsing settings isn't visible for me on themes, and so I was wondering:
is there anything still missing to close this issue? (e.g. do we have a test case for this scenario that runs on the HTML about:addons?).

It works. I've updated toolkit/mozapps/extensions/test/browser/browser_webext_incognito.js to test both XUL and HTML: https://phabricator.services.mozilla.com/D34277

Searchfox hasn't been updated yet, but the logic is equivalent to https://searchfox.org/mozilla-central/rev/153172de0c5bfca31ef861bd8fc0995f44cada6a/toolkit/mozapps/extensions/test/browser/browser_webext_incognito.js#375-390

The description wasn't clear, this is for dynamic themes that show up in the extensions tab, like Firefox Color. They're always allowed in private windows.

After discussing with Luca, this isn't related to dynamic themes but privileged extensions. The PERM_CAN_CHANGE_PRIVATEBROWSING_ACCESS permission can be used to check if the setting should be shown [1].

[1] https://searchfox.org/mozilla-central/rev/0da35261b6789eec65476dbdd4913df6e235af6d/toolkit/mozapps/extensions/content/extensions.js#2924

This is related to the test that I mentioned. I added two TODOs that both reference bug 1557792, but one of them could be referencing this one instead.

Basically, this bug can be resolved when the following TODO is addressed: https://searchfox.org/mozilla-central/rev/227f5329f75bd8b16c6b146a7414598a420260cb/toolkit/mozapps/extensions/test/browser/browser_webext_incognito.js#126

This is fixed by bug 1557792 along with the related message.