AFAIK there is no way to stop the updating. I will say this because they don't want user preventing this is to look on the web to find what your asking. That is the most that can be said about this.

Please see this support page. https://support.mozilla.org/en-US/kb/forum-response-tabs-below-url-bar

boudrell said

If it is Mozilla's intention to force people to accept "tabs on top" then say so and I'll migrate over to Chrome or the MS thing like so many others have done already.

That 'MS thing' (in my case Edge) has tabs on top ....

As for Chrome, please see :

https://productforums.google.com/forum/#!msg/chrome/w320FR18Wiw/XZePYzU9CQAJ

McCoy, Thanks for your reply. Neither Chrome nor Edge are familiar to me which is why I didn't mention them in my less than calm comments, for which I apologize. So now I will add this and ask for further clarification!

      Why is it necessary for Mozilla to change the userChrome.css file during any update when it appears to me that this is the user's customization file? If Mozilla needs to change the file, why not have the change added to the bottom of the text leaving the original body intact? Regardless, if an update is going to make changes to the file, that should be conspicuously displayed as a warning in the explanatory notes about the update.  
      Thank you!

My answer addressed your mention of "tabs on top" - the older "tabs on bottom location a feature that Mozilla removed from Firefox back in Firefox 29 with the then new Australis User Interface.

Then with the run of Australis users were using extensions such as the Classic Theme Restorer extension to "restore" the removed "tabs on bottom" option. Unfortunately Quantum eliminated the ability for extensions to modify the UI as those extensions had been doing. Too many users forgot that they were using an extension "to move the tabstrip" to the legacy location; and now those users are using a userChrome.css to effect that change.

The userChrome.css is an optional file that users choose to create and populate with the code to do customizations that can't be done in a different manner like with an extension, such as modifications to the location of the Tab bar.

Now with Firefox 65 there was a change made to Firefox as far the the code for the tab bar and the title bar - the update didn't tamper with the userChrome.css file which Mozilla doesn't even know exists - Mozilla changed the UI code in Firefox. That change broke the userChrome.css code that people have been using for "tabs on bottom". You decided to use that code, so it's up to you to fix it, which is what I pointed you towards.

That said, Mozilla is in the process of eliminating XUL from Firefox, a piece at a time over many versions by switching to HTML for the UI a piece at a time. So if you are using a lot of userChrome.css code, that code may get "broken" - sooner or later.

As far as "explanatory notes" about the changes with each new version which is released, do you even read the Release Notes?

Sure those Release Notes - https://www.mozilla.org/en-US/firefox/65.0/releasenotes/ and https://www.mozilla.org/en-US/firefox/65.0.1/releasenotes/ - don't specifically changes to the underlying code for the UI, but there is a website that covers upcoming changes made in the Nightly (alpha) and the Beta versions of Firefox which lead to the Release version of Firefox in less than 12 and 6 weeks. https://www.ghacks.net/category/firefox/

the-edmeister - You said, "....'the update didn't tamper with the userChrome.css file which Mozilla doesn't even know exists - Mozilla changed the UI code in Firefox."

Me - So what you're saying is that Mozilla either broke the userChrome.css or changed the UI putting the tabs back on top. Which is it? Give me your best guess. I did change the userChrome.css code to get back to "tabs on bottom" using the info you provided. It doesn't change the fact that I will have to do it all over again sometime down the road with another update.

You say, "......Mozilla is in the process of eliminating XUL from Firefox, a piece at a time over many versions by switching to HTML for the UI a piece at a time. So if you are using a lot of userChrome.css code, that code may get "broken" - sooner or later."

Me - The only code I know about or want is the "tabs on bottom". I have no idea what any other code does.

You say, "As far as "explanatory notes" about the changes with each new version which is released, do you even read the Release Notes?"

Me - The answer is yes, I read them, and they do not say anything about changing the userChrome.css or the UI to put the "tabs on top"

You say, "Sure those Release Notes - https://www.mozilla.org/en-US/firefox/65.0/releasenotes/ and https://www.mozilla.org/en-US/firefox/65.0.1/releasenotes/ - don't specifically changes to the underlying code for the UI, but there is a website that covers upcoming changes made in the Nightly (alpha) and the Beta versions of Firefox which lead to the Release version of Firefox in less than 12 and 6 weeks. https://www.ghacks.net/category/firefox/"

Me - To me you're implying that I should be that technically savvy or curiously nerdy to follow all of the coding efforts with the ongoing development of Firefox. That is definitely not me nor do I suspect the case with the majority of Firefox users. Mozilla was once so proud and enthusiastic about giving Firefox users greater control over their browser interface with more customization options but have continually failed to address this issue since it first arose. Unfortunately they now seem to make decisions about what and how to do things that alienate more and more users based on who knows what criteria. One has to wonder if they make changes based on ill-conceived ideas out of the blue or strictly on the personal whims of its programming staff.

edited to fix the leading spaces which throws this forum in to a "code" mode and off the right side of the monitor

Perhaps as you mentioned in your initial posting, using a different web browser might be best for you.