Multiple Windows, Single Session ：Turns out pressing Ctrl+Q or closing from the menu does this (and works better). (Relaunch closed windows on next startup. Experimental. Addon button closes windows for good.)..
Multiple Windows, Single Session 更新
Multiple Windows, Single Session 简介
I DO recommend closing your MAIN window LAST. Especially so, if you’re using something like Tree Style Tabs.
What it does
Basically, you close a window with the X and this addon will restart that window when Firefox reboots. If you want to permanently close a window, then use the addon button.
Turns out pressing Ctrl+Q or closing from the menu does this (and works better).
It’s written in purescript, which is honestly pretty terrible for this.(hindsight)