[wanone]

Worked for me in Firefox private browsing

[sebazzz]

Firefox may be leveraging container functionality for private mode. Firefox containers are fully functional isolated containers of data where regular browser features function like expected.

Even if it doesn't, the Firefox temporary containers add-on would help.

[gibybo]

Also worked for me in Chrome's incognito

[throwaway8129]

They are A/B testing it, or rolling it out slowly. I saw reports a few weeks ago, but didn't get hit with it myself until today.