Enable userChrome.css:
       Open Firefox and type about:config in the address bar and press Enter.
       Click "Accept the Risk and Continue".
       Search for toolkit.legacyUserProfileCustomizations.stylesheets.
       Double-click the entry to set it to true.

   Find Your Firefox Profile Folder:
       Type about:profiles in the address bar and press Enter.
       Locate the profile you are using (it’s usually the one marked as "Default Profile").
       Under the profile, click "Open Folder" (this will open the profile folder in your file explorer).

   Create chrome Folder:
       In your profile folder, check if a folder named chrome exists. If it doesn’t, create a new folder and name it chrome.

   Create/Edit userChrome.css:
       Inside the chrome folder, create a new file named userChrome.css if it doesn’t already exist.
       Open userChrome.css in a text editor (e.g., Notepad, Notepad++, VS Code).

   Add Custom CSS:

       Add the following CSS code to the userChrome.css file to remove the separator:

       css

       :root {
         --tabs-navbar-separator-color: rgba(0, 0, 0, 0) !important;
       }

   Save and Restart Firefox:
       Save the userChrome.css file.
       Restart Firefox to see the changes take effect.

So in the end i just ask chatGPT and it works... Step-by-Step Guide Enable userChrome.css: Open Firefox and type about:config in the address bar and press Enter. Click "Accept the Risk and Continue". Search for toolkit.legacyUserProfileCustomizations.stylesheets. Double-click the entry to set it to true. Find Your Firefox Profile Folder: Type about:profiles in the address bar and press Enter. Locate the profile you are using (it’s usually the one marked as "Default Profile"). Under the profile, click "Open Folder" (this will open the profile folder in your file explorer). Create chrome Folder: In your profile folder, check if a folder named chrome exists. If it doesn’t, create a new folder and name it chrome. Create/Edit userChrome.css: Inside the chrome folder, create a new file named userChrome.css if it doesn’t already exist. Open userChrome.css in a text editor (e.g., Notepad, Notepad++, VS Code). Add Custom CSS: Add the following CSS code to the userChrome.css file to remove the separator: css :root { --tabs-navbar-separator-color: rgba(0, 0, 0, 0) !important; } Save and Restart Firefox: Save the userChrome.css file. Restart Firefox to see the changes take effect. Summary By enabling the userChrome.css file, adding the custom CSS with !important, and restarting Firefox, you can permanently remove the border line between the text bar and the tab bar. This ensures that your custom style is applied consistently across sessions.