I had this problem twice: I'm closing Firefox, starting it again, but it won't start - that's because Firefox process is still active. After killing it and starting FF, all tabs are lost. Both times Firefox was going for some longer time, using about 1GB of memory, and there were suspends in between. Also I may have installed/deinstalled some add-ons. Since i use tab groups i have a lot of tabs and loosing them all is quite painful.