How do I put the tabs below the bookmark tool bar?
Several people have asked the same question. I am not technologically competent. A simple answer is required. The tabs have always been below the Bookmarks toolbar. It seems (and feels) totally illogical to have the tabs I am engaged with to be above the toolbar menu which is a secondary focus. Please fix this, give us an option to change it, or explain why you will not change it.
Your tabs were below your bookmarks toolbar because you used some extension that changed the interface of Firefox. Such extensions won't work anymore because of the changes mentioned here: https://blog.mozilla.org/blog/2017/11/14/introducing-firefox-quantum/
There's a workaround you may try to use, but I'm not sure if it still works or for long it will keep working: https://support.mozilla.org/en-US/questions/1032466#answer-656445