To add on to Anonymous user fc7bf5's comment. If you want to get rid of the Horizontal Tabs and the Title Bar for the sidebar simply add the following in your Firefox Profile. This is not the Local AppData one, but the Roaming one which is normally found at: %UserProfile%\Appdata\Roaming\Mozilla\Firefox\Profiles

You will have a profile folder here, maybe more if you utilize multiple profiles in Firefox. Once you pin-point yours you simply need to add a new folder called "Chrome" in your profile folder and create a new text file "userChrome.css" make sure it is a css file and not a txt. you may have to google how to change file extensions by revealing them in Windows. The contents of that file should contain one or both of the following segments depending on what you want.

/* Contributed by Anonymous user 099626's */
/* Remove horizontal tabs */
#tabbrowser-tabs {
visibility: collapse;
}

/* Remove sidebar header */
#sidebar-header {
visibility: collapse;
}

Once this is done you will have to restart FireFox. Depending on your theme in FireFox you may notice you can't see the windows control buttons in the upper right. To fix this simply go to the "Customize..." menu and check the "Title Bar" option then click done.

ps. I've noticed some FireFox menu's don't take the removal of the Horizontal Tabs into account due to it not being a build in option. With this in mind option screens like "Customize..." may take over your full screen including the TreeStyle Tabs. If this happens simply look for another way to close the screen, which should be provided.

pps. TreeStyle Tabs is a fantastic app that I've used for years, I've slowly started moving to Chrome because of the noticeable speed difference and the large amount of multitasking I have to do daily. I am excited to see if FireFox Quantum can live up to the speed it boasts. That being said...

Happy Tab Hoarding! :D