https://support.mozilla.org/en-US/kb/put-tabs-back-on-bottom

There is a "Tabs on Top" item in the toolbar right-click context menu and in "View > Toolbars" or "Firefox > Options" to place the tab bar at the previous position just above the browser window.
Current Firefox version only show the "Tabs on Top" menu entry if the tabs aren't in the default position on top.
If the tabs are on top and the menu entry isn't available and you want to move the tabs below the navigation toolbar then you need to flip the browser.tabs.onTop pref to false on the about:config page.

• http://kb.mozillazine.org/about:config

Come on man - this is a restriction on freedom. Why can't I just tick or untick something to suit my preferences. I can with everything else, what's so special about this - perfect place is the 'tabs' tab on 'Options'. I'm trying to USE firefox, not play with it or update it. Now my train of thought is completely interupted and I have to wait 25,632 years before cycling back to the same thought, or something like that. And the thought really was going to earn millions, and I would have donated a sizable proportion to the Firefox guys who are superstars in my opinion. But now, I have to readjust my entire mental make-up and get used to tabs on top, or mess with something I don't understand for a couple of hours. I think rebirth is better. You see things like this could cause suicide to people with a nervous disposition. Had Aaron just done a Firefox reset like I did to get rid of that horrible slider thing?

Kind regards

David

@dgwest7, I don't know if this will help you, but here goes. I'm the OP of this thread. None of their solutions helped me, as obviously the option is no longer available in the update. However, when I transferred my entire Firefox Profile (it's simply a directory under AppData I believe, let me know if you have trouble finding it), Firefox was put just back how I like it without the tabs on top (and with all my bookmarks, too!). Hope this helps.

You can always toggle the browser.tabs.onTop pref to false on the about:config page to move the tab bar to its down position.
A Firefox restart is necessary to update the menu entry and either show or remove it.