pusheen-black-no-borders per OfficialGoGo

If you have a white bar between the tabs and the search area do the following:
1) type "about:profiles" in the search bar
2) Go to "Root Directory" and click on "Open Folder" next to the directory.
3) Create a folder named "chrome" in this directory
4) in that folder create a file named "userChrome.css".
(make sure this is .css and not css.txt or something by enabling extensions for on windows (View, Options, Change Folder Options... , View , uncheck "Hide extensions for known file types"
5) Open the .css file with Notepad or any other text editor software of your choice
6) write the following in it:

/* Remove Line at top of nav bar */
#nav-bar {
box-shadow: none !important;
}

7) Save and restart Firefox.

If you still have the bar then:
1) Keep the files you created as they are
2) write about:config in the Firefox search bar
3) accept the risks
4) write "toolkit.legacyUserProfileCustomizations.stylesheets" in the search section.
5) Double click on "false" and set it to "True"

If you have any other issues, let me know.