Session Manager: is this built into Firefox now? It's very important that when I restart Firefox, I get the tabs and windows I had before. Can anybody who doesn't use an add-on for that confirm whether that works out of the box now?

Also, this seems to work for me without an add-on, but I can only do the exact session I had before (not an earlier one) and it fails if I have to hard restart the computer for some reason. It'll open up some much-earlier set of tabs, even if I've closed and opened Firefox between then and now. Weird, weird, so weird.