I don't know why Containers do not work in private windows. However, the purpose of private windows is to avoid writing data to disk, and it was not considered a general purpose browsing mode.

If you want to limit third party cookies in private windows, beyond blocking the tracking server cookies that are blocked in regular and private windows by default, you could switch to "Strict" mode Tracking Protection. As far as I know, this works everywhere in Firefox: See: https://blog.mozilla.org/security/2021/02/23/total-cookie-protection/

Thank you, I was going to quote the same article.

So by having "strict" selected or using an even more strict setting like in the image attached, cross-site cookies are technically stopped? therefore multi-account-containers add-on no needed?

Cheers,

Multi-Account Containers is still useful when you want to have multiple sessions on the same site, such as logging in to two Gmail accounts independently (those are first party cookies).

Yes, you are right: "strict/custom" mode only isolates third party cookies. Still not sure why multi-account-containers is not available in Private browsing (PB) mode. It is 100% an excellent addition on top of PB. Is there a special place to suggest this?

You could search in the bug tracking system to see whether there is a pending bug related to having container tabs in private windows:

https://bugzilla.mozilla.org/

See also: Bugzilla Etiquette

• https://www.google.com/search?sa=N&num=100&q=firefox%20containers%20private%20browsing%20mode
• https://www.reddit.com/r/firefox/comments/auberb/why_exactly_dont_containers_work_in_private/

I ended up not using private browsing (PB) anymore and getting as close as possible to PB in the settings by deleting history and cookies at exit. Then, I opted for a add-on called Temporaty Containers which works fantastic in conjunction with containers!

I am also paying close attention to OpenWorlf project.