You can have a different set of App tabs in each window, so they are not carried over to a new window.

Use "Firefox/File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit") to close Firefox if you are currently doing that by clicking the close X on the title bar to close all open windows at once.

An example - I am searching for holiday hotels so I search 'hotels in skegness' in my primary window. I then click on various hotels and view their own website. Mostly, the hotel's own website opens in a new secondary window (not tab) automatically: sometimes as a tab in my primary window: sometimes replacing what is the active tab: I have no control over it. So I end up with maybe 6 windows and 12 tabs, each with a different hotel to view and research. I then check each hotel and if not what I want, I close that tab or window down and move on. I don't want to use File>Exit because it closes everything. So it's very easy to close my primary window while secondary windows are still open, and that's when the pinnd tabs are lost. Programming-wise, all it takes is a parameter in a configuration file, so please fix it.