I really don't know what to make of it. I very rarely shut down the browser with tabs open in two different windows. When I do, I do expect them to open up that way when I start the browser again, and it does that just fine.

Sometimes, though, I only have one window open, and when I restart Firefox my tabs are out of order. Like I said in the title sometimes they come back in two different windows. It makes no sense to me.

I thought it had to have something to do with TreeStyleTabs, but nope. I disabled it and it still happens.

I'm on Beta 57.0. Windows 10 64, latest

Addons:

Bitwarden
Containers on the Go
Cookie Autodelete
Decentraleyes
Distill Web Monitor
Foxy Gestures
Google search link fix
HD Quality for YouTube
HTTPS Everywhere
Imagus
Moderator toolbox for Reddit
Multi-Account Containers
New Tab Tools
OneTab (from the Chrome Store)
Reddit Enhancement Suite
Screengrab!
Snoop
Swift Selection Search
Tab Suspender
uBlock Origin
Video Speed Controller
Violentmonkey
webcompat.com reporter