I doubt you will find a browser which does not have tabs at the top. GC has tabs at the top and no way to change it. The userChrome.css code for doing that has to be updated so you should go to the site where you got it and update it.

One reason that UI customizations can disappear in Firefox -- this one was removed in Firefox 29.0 -- is when the testing burden becomes too high. Many features have automated tests, but user interface features can't always be verified using automated tests, and if everything is design to align with the standard layout, going back and fixing dozens of little things with an alternate layout consumes a lot of resources.

Firefox 113 continued the process of standardizing the user interface code and removing legacy elements. The latest change removed some proprietary properties (-moz-box-*) that were previously used to change the order of elements (up or down). So as you know, you'll need to update your code again.

General suggestions:

(1) Return to your original source. The person/site that provided the original code (often on Github) usually has already updated to account for the changes. These include the MrOtherGuy repository and Aris' CustomCSSforFx repository on Github.

(2) Ask for help on the FirefoxCSS subreddit. If you are a DIY coder, check out the sticky post with the technical background explanation, and if you want more help, try reading through the latest threads and/or starting a new one.

=> Sticky background thread: https://www.reddit.com/r/FirefoxCSS/comments/11odffm/psa_incoming_changes_to_default_element/

=> Latest discussions on r/FirefoxCSS: https://www.reddit.com/r/FirefoxCSS/new/

What CSS code in userChrome.css did you previously use for the tabs below the Navigation Toolbar as it might be easy to fix it ?