I duped my old profile on Firefox to isolate my different use case. I duped it by overwriting everything from a new profile folder created with about:profiles by contents of the old profile folder. At first the pinned tiles I changed on the new about:home also changed the old profile. Then I found that extentions enabled in Private mode are also shared between the profiles and will leak through it. Just last night I found that even browsing history is added to both profiles without the other one caching favicons, deleting history does not affect the other one though. User configuration seems to be isolated though including toolbar layout and extentions can be disabled independently. I found this to only affect duped profiles as I created a fresh new one to test the above phenomenon and none of them are present. In addition to this machine, my GNU/Laptop also has the same issue with Librewolf and profiles are only duped from the respective browsers, so I think it is OS agnostic.