<profile-dir>/chrome/userChrome.css:

       #tabbrowser-tabs {
         visibility: collapse !important;
       }

It worked for several days but now the problem has returned.  I tried repeating the same trick on xulstore.json.tmp, problem persisted, so I unlocked both files and left them alone.

Hello, I'm currently on Gnome Shell in a fresh install of Ubuntu 17.10 with very little additional apps installed. I'm using Firefox Quantum. I'm using Xorg because Wayland is alot slower on my machine. I have a vertical tab addon called Tab Center Redux. In order to get rid of my now unwanted horizontal tabs, I'm using a css mod via <profile-dir>/chrome/userChrome.css: #tabbrowser-tabs { visibility: collapse !important; } This has resulted in a strange bug that appears to be related directly to the usage of that #tabbrowser-tabs line in useChrome.css: I use Firefox in Maximized mode but every time I restart it, instead of starting maximized, FF starts unmaximized and only a quarter of my desktop in size, it then gets dragged magically by itself from the center of the screen and then expands to the full size. Sometimes, it won't bother expanding and will stay unmaximized, sometimes it will get maximized but only a small fraction of the browser Window would actually be used and the rest would be whitespace or a distorted copy of what's in the center. I cannot get this bug to manifest consistently. Sometimes, it's happening all day, sometimes it's gone for a couple. Leaving only the css tab bar hack activated, I've tried uninstalling all my FF extensions as well disabling all my gnome-shell extensions. I logged out and logged back in to give the g-shell extensions a chance to go away. Problem still persisted. Deleted that line in userChrome.css and the problem stopped. Re-installed all my FF extension, enabled my g-shell extensions. Problem was still absent. Re-added that line to userChrome.css, problem returned. I tried brute forcing a solution by making xulstore.json immutable after editing it with the following with parameters: {"chrome://browser/content/browser.xul":{"navigator-toolbox":{"iconsize":"small"},"main-window":{"screenX":"0","screenY":"48","width":"1600","height":"852","sizemode":"maximized"},"sidebar-title":{"value":"Tabs"},"sidebar-box":{"sidebarcommand":"_0ad88674-2b41-4cfb-99e3-e206c74a0076_-sidebar-action","checked":"true","width":""}},"chrome://browser/content/history/history-panel.xul":{"viewButton":{"selectedsort":"lastvisited"}}} It worked for several days but now the problem has returned. I tried repeating the same trick on xulstore.json.tmp, problem persisted, so I unlocked both files and left them alone. Any suggestions? Are there any specific logs I can upload to help you?