Hiding the tab bar is no longer a builtin Firefox feature. This means that an addon or a third party program is hiding your tab bar from you.

To see if there is an addon that is creating this issue, please try Firefox in safe mode. Safe mode is a diagnostic mode that will temporarily disable your addons, turn off hardware acceleration and reset some settings. For more information on using Firefox safe mode, please refer to this Mozilla support article:

• Diagnose Firefox issues using Troubleshoot Mode

Does the issue still occur in safe mode? Let us know.

Type about:customizing<enter> in the address bar. Look for Show / Hide Toolbars at the bottom left.

There is also a Restore Defaults button at the bottom middle.