I'm not sure what you mean by delete, but I'll mention a couple things to start: (1) If you accidentally closed a tab but there is another tab still open in that same window: * Right-click any tab > Undo close tab ** Ctrl+Shift+t does the same thing * Use the History menu's list of recently closed tabs to re-open the tab (2) If you accidentally closed a tab, and that closed the whole window: * If you had another window open (Firefox didn't close): use the History menu's list of recently closed windows to re-open the window * If Firefox shut down: after starting Firefox, check the History menu's list of recently closed windows. If none are listed, try "Restore Previous Session" on the History menu and then check again. Was either of those was you're looking for?