Hmm, that setting usually works. Could you check whether either of these are available (not hidden or grayed out): * "3-bar" menu button > History > Restore Previous Session * (menu bar) History > Restore Previous Session If they are missing or grayed, check either: * "3-bar" menu button > History > Recently Closed Windows * (menu bar) History > Recently Closed Windows Can you find your missing windows/tabs that way? ---- ''If there is nothing there:'' It could be a problem with browsing history not being saved (or being cleared using settings). ''If you found the missing windows/tabs:'' I wonder whether your Firefox is not quitting all the way. Are you using one of these to close Firefox: * "3-bar" menu button > Quit * (menu bar) Firefox > Quit If not, Firefox might still be running in the background, and just launching a new window in an existing session rather than starting up and then restoring your previous session.