hello, you can use the following extension to work around it: https://addons.mozilla.org/firefox/addon/hide-tab-bar-with-one-tab/

THANK YOU!! This solution should have been TOTALLY unnecessary. I do not appreciate having my personal selections changed because of an update. I expect updates to deal with issues "under the hood" that I don't deal with on a daily basis. It is very arrogant and rude to change my personal preferences. When I take my car in for an oil change, that's ALL I want done--update my oil. I did not give you permission to change the layout of my car's dashboard, and I would be VERY HIGHLY annoyed with they them if they DID move my speedometer to the left, put the radio controls in the center, or move the fuel gauge to the opposite corner. And yet, this is EXACTLY what you did to me.

At the very most, do this: In the HELP menu, add a new BY THE WAY section. It then opens up to a message "Some people have found these options to be useful. Click one to check it out when you have some free time." Then list the various layout options available. At the end of the "test drive", pose the question, "Would you like to keep this option?" Then give 3 answer choices: YES, NO, and NO, DON'T ASK AGAIN.

You can suggest that BY THE WAY feature to Mozilla thru Help > Submit Feedback.