Firefox: quick and simple way to close current window and open new window?

In the Firefox webbrowser:
Is there a quick & simple way to close the current window and open a new window instead?

(Sidenote: For the purpose of this question, there is only 1 currently open window.)

What I am currently doing:

1. initial state (focus is on this window)
2. CTRL+N: open new window (focus is on new window now)
3. ALT+Tab (in Windows): to switch back to the old window (focus is on old window now)
4. CTRL+W: close old window (focus is on new window now)

Is there a more efficient way?
I am working on an old computer and the focus switch old window --> new window --> old window --> new window is lagging and consumes a lot of time.(And even if I had newer hardware available, I'd still want to operate as effectively as possible.)

The perfect solution for me would be to click on the "X" button to close the window and have Firefox closing the window and NOT quit/exit but opening a new window instead. This would require an imaginary about:config setting like browser.quitAppWithLastWindow=false by analogy to the existing about:config setting browser.tabs.closeWindowWithLastTab=false. But I assume there is no such setting?

I'd prefer a solution with built-in means (without adding an external program).

PS: In case someone asks, why I don't just close all tabs of the current window (CTRL+SHIFT+W) and be done with it?

1. because in my experience opening a new/fresh window (instead of continuing to work in the same window) frees up more RAM
2. because I want to have the benefits of a new window, e.g. CTRL+SHIFT+T doesn't open a closed tab from the previous (closed) window, which I don't need anymore