It seems firefox 4 has stupidly removed the option to save tabs on exit however there’s a way to get this very useful functionality back. Here’s how:
- Type about:config in the URL/Location/Address bar and press the Enter key
- Accept the warning message (promise to be careful)
- The preferences page/list will open
- Filter = browser.tabs.warnOnClose
- If its value is set to false, double-click on it (in the lower panel) to toggle its value to true
- Repeat steps 4 and 5 for these following 3 preferences also:
Taken from mozilla support. A big thank you to user Helper7677 for this solution!