Firefox has updated and once again the Tab Bar is on top!

Why they keep doing this is beyond me. It is damned irritating.

The fix is detailed HERE.

In case that link disappears the technique and code are as follows.

Locate your userChrome.css

• First, you need to enable the userChrome.css file. If you’ve done this before, you’re probably already muttering, “Here we go again…”
• Click ‘Help’ > ‘More troubleshooting information’ > Profile folder – ‘Open folder’
• Navigate to your Firefox profile folder.
• Inside, create a folder named chrome if it doesn’t exist.

Add the Magic Code

• Inside the chrome folder, create a file named userChrome.css. (Feel free to mumble, “Thanks for the extra work, Mozilla.”)
• Paste the following code into your userChrome.css file:
• SAVE and exit Firefox.
• Restart Firefox

Celebrate Victory

The Code:

/*Additions by Codehaven - Mike to change specific sizes*/

/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/tabs_on_bottom_v2.css made available under Mozilla Public License v. 2.0
See the above repository for updates as well as full license text. */

/* This reorders toolbar to place tabs below other toolbars. Requires Firefox 133+ */

@media (-moz-bool-pref: "userchrome.force-window-controls-on-left.enabled"){
  #nav-bar > .titlebar-buttonbox-container{
    order: -1 !important;
    > .titlebar-buttonbox{
      flex-direction: row-reverse;
    }
  }
}
@media not (-moz-bool-pref: "sidebar.verticalTabs"){
  .global-notificationbox,
  #tab-notification-deck,
  #TabsToolbar{
    order: 1;
  }
  #TabsToolbar > :is(.titlebar-spacer,.titlebar-buttonbox-container){
    display: none;
  }
  :root[sizemode="fullscreen"] #nav-bar > .titlebar-buttonbox-container{
    display: flex !important;
  }
  :root[tabsintitlebar] #toolbar-menubar:not([autohide="false"]) ~ #nav-bar{
    > .titlebar-buttonbox-container{
      display: flex !important;
    }
    :root[sizemode="normal"] & {
      > .titlebar-spacer{
        display: flex !important;
      }
    }
    :root[sizemode="maximized"] & {
      > .titlebar-spacer[type="post-tabs"]{
        display: flex !important;
      }
      @media (-moz-bool-pref: "userchrome.force-window-controls-on-left.enabled"),
        (-moz-gtk-csd-reversed-placement),
        (-moz-platform: macos){
        > .titlebar-spacer[type="post-tabs"]{
          display: none !important;
        }
        > .titlebar-spacer[type="pre-tabs"]{
          display: flex !important;
        }
      }
    }
  }
}

/* Adjust the height of the Tabs Toolbar */
#TabsToolbar {
    height: 32px !important; /* Neater toolbar height */
    min-height: 32px !important;
    max-height: 32px !important;
    margin: 10px 0 -5px 0 !important;
    padding: 0 !important;
    font-size: 14px !important;
}

/* Adjust tab height to match the toolbar */
#TabsToolbar .tabbrowser-tab {
    height: 36px !important; /* Matches toolbar for a balanced look */
    padding: 0 4px !important; /* Adds slight spacing between tabs */
}

/* Center align tab content */
#TabsToolbar .tab-label {
    line-height: 34px !important; /* Align text with tab height */
    margin: 0 !important;
}

/* Smaller close button for a cleaner appearance */
#TabsToolbar .tab-close-button {
    height: 14px !important; /* Smaller close button */
    width: 14px !important; /* Proportional width */
    margin: 0 2px !important; /* Adjust spacing around the button */
    padding: 0 !important;
}

/* Prevent overflow and ensure neat alignment */
.tabbrowser-tab .tab-stack {
    margin: 0 !important;
    padding: 0 !important;
}

/* Adjust tab icon size */
#TabsToolbar .tab-icon-image {
    height: 18px !important; /* Matches the smaller close button */
    width: 18px !important;
}

Incidentally, There was already a userChrome.css file [probably from a previous fix] so I replaced the entire contents with the above code.

It works!