Running Firefox on 2 computers

I run 2 computers, 1 at home and 1 at work. Up to V3.5, I used to sync FF with my copy on a USB drive and take it to the other computer and sync it to the other computer. This has worked ok. Now that I have upgraded to V4.0.1, this method no longer works. Firefox launches, stays in the task bar and can only be maximized.
The main reason I am doing it this way is that I want to continue to work with tabs I had before. I could make this all work if I knew the file that contains the tabs. This way I would only have to sync that file and continue to work.
Could someone please let me know which file in V4.0.1 contains the tabs?
Thank you.