The user may want a custom new tab page in private windows which would require overriding about:privatebrowsing. Chrome allows extensions that override the new tab page to work in incognito windows.
Adding Javaun, as guidance around this would be needed. Third-party overrides would potentially allow info leaks and tracking in PBM, so we'd need to flesh this out from a Private Browsing PoV first.
This has landed in bug 1234150 and currently this works in private browsing mode as well as normal mode by allowing you to override the new tab page. I'm confused by comment 0 which contradicts the documentation "New Tab pages cannot be overridden in incognito windows" https://developer.chrome.com/extensions/override Should we be restricting this in private browsing?
Per comment #1, looking for guidance from private browsing product. My vote is yes. Custom newtabs can leak info to orgs other than mozilla. Users should be required to specifically enable an addon in private browsing mode to effect the changes they make.
(In reply to Kev Needham [:kev] from comment #3) > My vote is yes. Custom newtabs can leak info to orgs other than mozilla. > Users should be required to specifically enable an addon in private browsing > mode to effect the changes they make. And, to be clear, our newtab does not, iirc, leak info by default, where other newtabs can (and frequently do) load resources from third party sites.
Changing bug title to be clearer that we are planning on removing this ability. This ability was added in Firefox 53, so we'll need to land this patch soon to remove it - based on Javaun's feedback.