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Ctrl+⇧ Shift+T (Windows) or ⌘ Command+⇧ Shift+T (Mac). This will quickly reopen the last tab that you closed. You can continue to press this to keep opening closed tabs in order. Your recently closed tabs will be displayed in the "Restore Closed Tabs" section. Clicking an entry will open it in a new tab, or you can reopen all of them at once by clicking "Restore Closed Tabs".
Press . Click the Firefox Menu button (☰) and select "History".