EDIT: This was this bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1081135 Updating to 36.0a1 fixes the problem What I have is a mostly default Nightly (35.0a1) installation With e10s enabled and "When Nightly starts" set to "Show my windows and tabs from last time" At first it was fine, upon starting nightly it would only open 1 window Then after a session where I had another window open where I closed the 'main' nightly window before the other one, it started happening. The next time I opened nightly, it would open my previous session and have another with the same tabs (So basically I now have 2 windows with the same tabs). Then closing the one window, browsing the net and then finishing the session. Next time I opened nightly, It then opened the 2 windows from last time and creates another one (All have the same tabs still). This goes on until I restart the system. Upon restarting, it would go back to 2 windows and so on. And the behaviour persists even after uninstallation of nightly, even deletion of the sessionstore.js or the whole profile folder. I've even tried disabling e10s to no avail. This didn't happen before, I've always had it set to "Show my windows and tabs from last time"