STR:

1. Enable permanent private browsing mode (PBM) by choosing Never remember history (browser.privatebrowsing.autostart = true).
2. Install an extension.

Expected:

The extension is available — button is visible in toolbar, preferences tab appears etc.

Actual:

The extension is listed as enabled but is unavailable, have to go into extension settings and allow in private windows to get it working. Since all windows are treated as private windows in PBM, this extra step is unnecessary and serves no purpose other than to cause confusion.

Workaround is to set extensions.allowPrivateBrowsingByDefault = true.

A post-install panel allows the user to check the private browsing option during the install flow. Access is intentionally denied unless the user explicitly gives permission.

allowPrivateBrowsingByDefault will be removed in a future version and should not be suggested as a workaround.

According to bug 1531976,

when in PPB, if you install an extension, we current auto-grant permission.

This is not true. I have recently had to test add-on installations with PPB enabled, and can't help but feeling that the user experience is terrible. Users enable PPB for various reasons, and a non-neglibile subset of them do so by toggling "Never remember history", without being aware about the implications on add-ons. When I absentmindedly dismiss the post-install doorhanger, it takes some effort to find the add-on that I had just installed and allow it access to private browsing mode. When a user consciously takes steps to install an add-on in the browser, they clearly want to use it.

I think that it would make sense to enable add-ons installed in PPB by default. WDYT?

What happens for a user who normally does not use PPB, enters it, installs an addon, then exits PPB? We decided that we should not enable addons in private mode by default for that reason. I think that bug 1531976 needs to be addressed.

The comment "we current auto-grant permission" mentioned above is taken out of context. Later in that paragraph is " Is this how we want that to work?"