I discovered this Extension for Firefox about a week ago, and it is a must. Here is a short description of what it does.
“SessionSaver restores your browser -exactly- as you left it, every startup, every time. Not even a crash will phase it. Windows, tabs, even things you were typing — they’re all saved. Use the menu to add + remove sessions; right, shift, or middle-clicking will delete. “Simple mode” for peace of mind, or “Expert mode” for advanced flexibility. Just Click. Install. Rad.”
It has saved me a few times since I have installed it. Check it out here.