I've had a short empty space in Firefox 29 and later. My tabs do not slide up into that area (the unused title bar space) until I maximize the window. Perhaps someone knows a trick to have them slide up all the time, if that's what you're looking to accomplish.

Try a negative margin-top for the Tab Bar (#TabsToolbar) with code in userChrome.css

Add code to the userChrome.css file below the default @namespace line.

• http://kb.mozillazine.org/userChrome.css

@namespace url("https://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

#TabsToolbar {margin-top:-1px!important}

The customization files userChrome.css (user interface) and userContent.css (websites) are located in the chrome folder in the Firefox profile folder.

• http://kb.mozillazine.org/Editing_configuration

OK, I can see now that the only time there is no space is when the window is maximised. Another question now presents itself - why don't I have a chrome folder in my profile?

The chrome folder and the userChrome.css and userContent.css do not exist by default and need to be created.

• Create the chrome folder (lowercase) in the <xxxxxxxx>.default profile folder if this folder doesn't exist
• Use a plain text editor like Notepad to create a (new) userChrome.css file in the chrome folder (file name is case sensitive)
• Paste the code in the userChrome.css file in the editor window
• Make sure that the userChrome.css file starts with the default @namespace line
• Make sure that you select "All files" and not "Text files" when you save the file via "Save file as" in the text editor as userChrome.css. Otherwise Windows may add a hidden .txt file extension and you end up with a not working userChrome.css.txt file

There is a pref (about:config: browser.tabs.drawInTitlebar) that controls the position of the tabs in maximized screen mode, but there is no setting for the position of the Tab bar when the screen is not maximized.

All plausible, but didn't change anything. I checked for the right directory, checked it was the correct case, checked it was a .css - but didn't make a difference!