1
Click the Start Menu, type "%appdata%," and press Enter. A new window appears.
2
Open the "Mozilla" folder. Open the "Firefox" folder. Open the "Profiles" folder. Open the folder corresponding to your Firefox user profile, choosing the one that ends with ".default" if you're unsure.
3
Open the "chrome" folder. If it does not exist, press Shift-Ctrl-N, type "chrome" and press "Enter."
4
Open the file "userChrome.css" in the "chrome" folder in a plain text editor. If you don't see the file, click the Start menu, type "notepad" and press Enter.
5
Add the following CSS code to the "userChrome.css" file or your new blank document:
.tabbrowser-tabs{
border:none !important;
}
6
Save the "userChrome.css" file. If you've created it from scratch, save it in the folder you created in Step 3.
7
Close and relaunch Firefox. The tab bar border is gone.