Are you using specific code in userChrome.css to move the Tab bar (I'm not seeing this bar in your screenshot) and possibly make other changes to the navigator-toolbox area or use extensions that make such modifications?

Those rules may have side effects. That line is either a border or a text shadow. See this file for CSS code for this area:

• chrome://browser/skin/browser.css

The bookmarks and tabs are underneath, not shown in the picture.

I've never customized the UI manually, but I have been using Classic Theme Restorer ever since the Google Chrome revamp.

I could possibly fix this myself if I could find any CSS files to edit -- should I copy this browser.css somewhere and let it override the built-in one?

Also, can I possibly use the Web Developer and/or Firebug style point+click interface to locate the UI elements? I'm not very good with CSS, you see.

The developer of Classic Theme Restorer maintains a support thread over here: http://forums.mozillazine.org/viewtopic.php?f=48&t=2827985

Sorry I can't help you myself, I am still using Firefox 28 while I try to Customize the 29-plus versions without resorting to using CTR. I have tried it, but I prefer doing my own "cobbles" rather than rely upon an extension that "restores" much what I didn't like about Firefox 4.0 thru 28 and overrides what I do like about Australis. IOW, CTR 'fixes' things about 90% opposite of what I would do, if I had the talent to do a "retro extension".

IMO, the orange Firefox button was the worst thing that Mozilla has ever done to Firefox. The Menu bar worked fine before Firefox 4 came out and still does, it just doesn't have be showing all the time = Compact Menu and its' derivatives over the years was the 'ticket' for me starting from before Firefox was even named Firefox.

See also this mozillaZine forum thread.

• http://forums.mozillazine.org/viewtopic.php?f=7&t=2777255

Okay, so apparently style customizations go in userChrome.css and userContent.css in ${mozilla_profile_directory}/chrome.

As for the black line,

 #TabsToolbar:not([collapsed="true"]) + #nav-bar {
   border-top: none !important;
 }

(userChrome.css)

gets rid of it, though not the gray line it leaves behind nor the gradient of the navigation bar. (Both are fine for me, and I'm much happier now that I know how to tweak the UI)

For the sake of reference,

 chrome://browser/content/browser.xul

gives me an inspectable rendering of the UI (though the original one, without my overrides)