If you want your Firefox windows to look like native GNOME/GTK windows (with rounded corners on all four sides):
1. In the location bar, enter:
about:config
2. In the search, enter:
rounded
3. Set the value of widget.gtk.rounded-bottom-corners.enabled to true
@aral and don't forget about Firefox Libadwaita theme to more native look
https://www.omgubuntu.co.uk/2022/07/firefox-gnome-theme-libadwaita-update
@aral What if I want my window to be hairy like a gnome?
@evilroda This is Linux, I’m sure someone’s already found a way to do it.