I usually open several tabs in FF and if I don't finish reading / using them I close the browser through the "windows task manager" by pressing the "end task" button.
After re-launching the FF browser I get this "Restore Previous Session" tab from which I continue the surfing.
Due to a reason described below this tab is gone but, I did saved A few days ago the "restore previous session" tab, but now I can't open it. I have the HTML file (.xht) and the folder that gets saved together with it.
It all happened because I gave my permission to a family member to use the Internet on my computer. Although being asked to surf through the Chrome Browser she opened the FF, closed it using the regular X button, re-opened it later and than closed it again - the "Restore Previous Session" can't be used in this case.
Ummm, use the menu. History->Restore previous session. The only reason it was working for you before is because using the task manager force quits the program. In this case firefox thinks the program crashed and is asking if you want to restore the previous session. Use the close button or use ALT-F4, not the task manager to close programs.