Firefox has an extension called watch in mpv which allows you to click on a given video or a button for the page and open the video in mpv. If your window manager allows tabbing various apps together you can even keep this in its own tab with firefox in the other.

Being its own app you can of course have this window floating and on top of your firefox window off in the corner. You could even designate a hotkey to move and resize this window to that position if you so desired.