Rated 5 out of 5 stars
Have just started using this along with the "Movable Firefox Button" extension to take better advantage of screen space and gain a look and feel similar to Google Chrome.
Took me a while to find this because I was searching for " remove the space above the tabs in Firefox ". Hopefully this comment will help someone else who is searching for the same thing to find this review and so find the add-on.
[Above text copy/pasted/amended from comments I have made elsewhere on the internet]
The extension isn't feature packed, but it does what it was designed to do. Full marks.