Currently, browser notifications show on [either your main display or just the active one, I'm not sure] but I want them to appear in my firefox window. I have Firefox open on a second monitor and I play games on my main monitor with Windows' "Focus Assist" turned on, but browser notifications ignore this and can interferre with my games (in a game-breaking way) when they pop up on my main monitor. Having them obey Focus Assist would be nice, sure, but I actually want those notifications to show up while I'm playing, not be completely hidden - I just don't want them on my main monitor. Hence my idea to show them in the browser window - which TBH seems like the obvious location for them in the frist place.
TLDR: Make browser notifications show in the browser window just like the completed downloads pop-ups.