Never mind, I found it: https://github.com/MrOtherGuy/firefox-csshacks/blob/master/chrome/tabs_on_bottom_v2.css Just replaced the contents of my userChrome.css with this one

Hi, The people who answer questions here, for the most part, are other users volunteering their time (like me), not Mozilla employees or developers. If you want to leave feedback for developers, you can go to the Firefox ''Help'' menu and select ''Share ideas and feedback…''. Alternatively, you can use [https://connect.mozilla.org/ this link]. Your feedback gets collected by a team of people who read it and gather data about the most common issues. You can also file a bug report or feature request. See [[File a bug report or feature request for Mozilla products]] for details.