Try PermaTabs, click here (Firefox's built-in feature works OK for me btw).
If no joy, try Avant Browser click here (piggybacks on the IE engine, but much better), which has a feature to re-open closed tabs on startup. On startup, it also gives you list of sites to chose from the last time, so you can untick the ones you don't require. I wish Firefox had this feature - may be there's an add-on somewhere! G