Starting with Firefox 63.0, the Ctrl+Tab shortcut now displays thumbnail previews of your tabs and cycles through tabs in recently used order. This new default behavior is activated only in new profiles and can be changed in preferences.
If you enable Ctrl+Tab thumbnail previews, pressing the Ctlt+Tab keys in Firefox displays thumbnail previews of all open tabs. This is like the Alt+Tab feature in Windows. You would press and hold the Ctrl key, tap the Tab key until you select the tab you want to switch to, and release the Ctrl key.
If you disable Ctrl+Tab thumbnail previews, pressing the Ctlt+Tab keys in Firefox only cycles through tabs in recently used order in Firefox without thumbnail previews. This was the default way before Firefox 3.0.
This tutorial will show you how to enable or disable pressing Ctrl+Tab keys displaying thumbnail previews of tabs in Firefox for your account in Windows 7, Windows 8, and Windows 10.