- Enable permanent private browsing mode (PBM) by choosing
Never remember history (
browser.privatebrowsing.autostart = true).
- Install an extension.
The extension is available — button is visible in toolbar, preferences tab appears etc.
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.