It is a trick because the tabs APPEAR to be loaded but they are not. That is called a "trick". There is zero point in having tabs restored on starting Fx or SM unless they are actually all restored when you start the browser. Otherwise, you should just go back to IE 6 type browsing. No point in tabs and certainly no point in restoring tabs. You might as well tell the browsers to not manage your sessions.
I have used Session Manager ever since development on it started ...many years ago. I use it on both Fx and SM. I don't see this crazy behavior in either browser. I have Fx 10 ESR and Fx 4.01 on my host machine. I still think it is far superior to Fx 10 and Fx 1.5 is the finest of Fx. After that it has been all downhill. I have SM 2.10.1 and I don't see this on it. I see the behavior on Opera 12 though.