Pinned Tabs (or App Tabs) allow you to keep your favorite web apps, like Facebook and Gmail, open and just a click away. This article explains how to use them.
I just discovered that you can pin tabs, which means that they are always loaded when you open firefox. This is a game changer for me. Whenever I open firefox, all my most visited tabs are automatically loaded and there's no loading time anymore. Of course I still need rules for cookie autodelete but it's awesome!
It works great in combination with the keyboard shortcuts for opening the first, second, ... eighth tab, which is Alt+1 (or +2, +3, etc.) for me, but I think is Ctrl or Cmd instead of Alt on other OS's.
However, they disappear and are not loaded anymore if you have multiple Firefox windows open and the window with the pinned tabs is not the last Firefox window one you close...
The pins are part of the window, so.. You can access old closed windows through the history menu, which I believe works after starting a new session after quitting it.