Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the add-ons is causing the problem (switch to the DEFAULT theme: Tools > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.

See:

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

If it does work in Safe-mode then disable all extensions and then try to find which is causing it by enabling one at a time until the problem reappears.

• Use "Disable all add-ons" on the Safe mode start window to disable all extensions.
• Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

Thanks Cor-el. I was hoping for an easy fix - ie someone else had already had the same problem and worked it out. Let's see if I am Robinson Crusoe.

Are you clicking on the web page or right clicking on the tab bar?

"Pin as App Tab" is only available if you right click the tab bar.

cor-el, thanks for the hint. I was clicking in the TAB bar with no result. I reloaded FF and the option is now back. So not sure what the cause was. Thanks anyway for your assistance.