I open and close Firefox many times in a day, usually without restarting computer. At some point it has decided to open/show all tabs I had open in previous session, which is different behaviour from before. I don't wan't it to remember what I had open. And it did not, until, well, I don't know. Recently. I haven't changed any settings. It is annoying. And because I have not checked "Open previous windows and tabs" in Settings/General/Startup, it seems like a bug.