The following post is for Macbook however for windows and linux users steps remains the same.
Go to Navigation sidebar with three dashes in the top right corner
Click on it.
Go to Help
Go to Troubleshooting information
You get an about:support page.
Go To location pointed by Profile Folder - Use Show in Finder for macbook.
Now - Create a folder - "chrome" inside that profile folder
Create a new file named userChrome.css in chrome folder
Paste this content
/* hides the native tabs */Close firefox and reload.
/* leaves space for the window buttons */