Mozilla moved the 'Tabs to the Top' almost ten years ago, and did once offer the option to have than at the bottom for a few years. That is long dead and for many years there was a few different extensions for Firefox that are now gone. And doing that reliquaries the CSS scripting that is what your provided screenshots does.

Mozilla refuses to make that change an option, so long after they totally removed the option that was only meant to be a temporary option to begin with.

Use the URL at the top of that for Github and locate the latest version of that script.

I would copy and paste that URL for you to help you, but a person can't copy text from an image.

I think this is the url you meant: https://github.com/aris-t2/customcssforfx#how-to-modify-custom-user-styles

However, there is nothing I can see on there that addresses moving the tab bar below the address and bookmarks bars. Any further help would be greatly appreciated.

You obviously know a lot about Firefox. Do you have any idea why they would refuse to make a simple choice for users? I see many people with the same request. Just wondering.

Thanks.

A "simple choice" was very complicated, especially considering the number of years that "choice" was extended long after it was finally removed from Firefox. An extension developer quickly came up with a Retro look, which haunted Mozilla for too damn many years due to Bugzilla reports and an extreme amount of support time that was "wasted".

"Executive decision" by Mozilla, after close to 8 years of issues with how much extension developers like Aris was affecting the UI and the subsequent support problems that landed in Mozilla lap, with Fx29 Australis (when the Firefox 4+ "menu option" for Tabs on Bottom was removed with the Fx29 Australis UI) and then up to Firefox 57 Quantum when the Add-ons coding was changed drastically and severely restricted as to what was available to extension developers. Firefox 57 was when Aris had to switch to supplying userChrome.css code via Github for his defunct Classic Theme Restorer extension. And support for that userChrome.css overloads this forum to this day. That code is constantly getting changed by Mozilla to effect a totally "new Firefox" in many, many stages behind the scenes (underlying code for the UI to draw the screen inerface) and it requires all too frequent patching by each user of that userChrome.css code manually of via Copy and Paste of each individual file. And god forbid that the user thru all their modifications into one userChrome.css file, which can make it very difficult to fix when "something breaks". That's when I reevaluated how I was using Firefox and how I was modifying the UI to suit my desires.

You just aren't looking deep enough there:
https://github.com/aris-t2/customcssforfx#downloads-for-firefox-quantum-60
Open that URL # section and see the hyperlinked.

I am on the verge of reaching 71 yo, but I adapted to Top Tabs quite easily, by like Firefox 10 or Fx11.

Yes, I am an experienced Firefox user, since Aug 2002 {I was 53 yo then} long before Firefox became the current name of this web browser (Phoenix first 6 [iirc] pre-release Alpha versions; then Firebird for 2 Alpha versions 7 & 8; then Firefox 0.9 for Beta testing {trademark issues over the "name" of the program}).

I first started with the optional userChrome.css file and barely used more that 20 or 25 simple preferences as late as Firefox 29 or so. Anything more I used an extension to modify the UI. But now it's gotten crazy! And there is a Bugzilla report that was never acted on to block userChrome.css modifications completely, a few versions into the Quantum era the lead developer decided to not do that; but the Bug is still open.

My bottom line is adapt to the Tabs being at the Top of the UI or IMO, use a different web browser that still has that "tabs directly above the page content area". I did that myself a number of years ago and tried like 4 different, separate versions that were built and modified using the Mozilla Firefox Open-Source code. They worked fine, but I was concerned about them being continued and with security as the older code the they were using became "stale" and (if or could THOSE developers) continue to "piggy-back" on Mozilla security updates and properly rewrite those patches back to the older version of Mozilla / Firefox code??
Risk vs reward !
I have used or "tried out the fit" of many other web browsers over the last 22 years that I have been "on the internet", and kept returning to Firefox to use as my primary web browser since like 2004 when Firefox hit the release 1.0 version.

Also, I can't help you much further beyond giving you this URL - https://www.reddit.com/r/FirefoxCSS/ - where there is very much going on with userChrome.css in Firefox. There are far more Firefox users over @Reddit than who come here, which upsets me when I see so many Firefox employees / developers posting over there (by their Reddit "badges" which are scrutinized by Reddit moderators for validity before accepting) doing support over there - https://www.reddit.com/r/firefox/ - and this support forum can't seem to get the help that we need from "staff" - when we ask specifically for almost every thread, in some instances. This forum is a shadow of what it once was; and the 2008 start up of this forum all but killed the independent mozillaZine fora that was started when Mozilla was created and received the Netscape browser code to take into the OpenSource arena. I learned my stuff @mozillaZine fora where I was very active for close to 7 years, although I did participate here @SUMO from almost the beginning of the Beta stage of SUMO development, I spent more time @mozillaZine until we were "waiting in line" to provide support and SUMO really needed more "warm bodies" who knew what they were doing with Firefox support. I now come here, yet, and visit mozillaZine almost everyday, and do weigh in @Reddit occasionally.

Just my opinions and reflecting my knowledge or perception of the situation and history of Firefox, and using my limited knowledge ...

As I step step off my soapbox ....

minor word context spelling to correct