Please see these 2 recently posted threads related to the same (or very similar) issue: how make it remember tabs when close/reopen firefox saved tabs opens as untitled

-) I hope this helps you solve it.

Please let us know how above works out, and if we can be of further assistance. Detailed feedback is appreciated and helps us to help you better, plus enables us to better help other users with similar issues.

I can recommend the following add-on which can take over the session management in Firefox. In addition, it is highly configurable and by all accounts very stable, so this might just be the solution for you: Session Manager add-on on addons.mozilla.org

Again, please provide feedback to let us improve the quality of the support.