Earlier quoted context omitted.
Ironically the whole point of it originally was sandboxing, and it’s true at least on iOS. Thus, you won’t be logged into the same sites within an in-app browser, and clicking a link from within an app (whether it appears to be an link or not) can’t automatically connect you to cookies and any other tracking from your actual browser.
On android I have firefox-focus as my default browser (and disable any in-app browsing) for that same purpose.
Settings > Advanced > "Open links in apps"
https://support.mozilla.org/en-US/kb/set-firefox-android-ope...