If you click on the link provided, you get:
"Firefox has lost its window frame, looks like a super zoomed in window without any bars"
Your concern, is “Lack of Window Frame . . .”
So there was some possibility, that the article (linked) might cause you to investigate.
The meaning of “Internet browser window frame”, varies. I much preferred the window frame style that included useful horizontal and vertical scroll bars and controls - because they made clear to the user/viewer at least the right and bottom boundaries of the browser window.
That very useful tooling was thrown away by new-age style=“minimalism” . . . that keeps getting rid of things that work[ed].
Things that include what has mysteriously disappeared from your Brave Browser (Windows OS), installed.
One thing you might try, is a command line startup - take a look at the steps: