Using the user.js from latest git, I am getting two firefox windows each time I start firefox. Removing the user.js solves the situation.
I've tested this on two separate computers with a clean install of Firefox.
https://github.com/pyllyukko/user.js/issues/288
https://github.com/pyllyukko/user.js/issues/292
https://github.com/pyllyukko/user.js#known-problems-and-limitations:
Clearing open windows on Firefox exit causes 2 windows to open when Firefox starts
https://bugzilla.mozilla.org/show_bug.cgi?id=1334945
Set privacy.clearOnShutdown.openWindows to false
Yep. Firefox bug 鈽癸笍
Most helpful comment
https://github.com/pyllyukko/user.js/issues/288
https://github.com/pyllyukko/user.js/issues/292
https://github.com/pyllyukko/user.js#known-problems-and-limitations:
Set privacy.clearOnShutdown.openWindows to false