Firefox Pinned Tabs
However, sometimes for some inexplicable reason, the pinned tabs disappear after shutting down Firefox and starting again. They are supposed to start automatically if you have selected “Restore Previous Session” in Settings->General.
Restore Your Pinned Tabs
How go get them back ? Simple, just like any other Window or tab that has been closed, you can reopen them through the History menu. Open the History menu (if the Menu is not showing right click on the top bar and select “Menu Bar”). From there, click on “Recently Closed Tabs” or “Recently Closed Windows”. Find your lost tab or window and click to open.