Use Tree Style Tabs Instead of the Default Tab Bar in Firefox

First install Tree Style Tab plugin.

Now for the tab hiding part:

1. Go to your profile directory, directory’s path can be found by going to about:support and open the Profile Directory
2. Create a directory called chrome
3. Create a file called userChrome.css
4. Put these in the file

#TabsToolbar {
  visibility: collapse;
}

1. Go to about:config and set toolkit.legacyUserProfileCustomizations.stylesheets to true
2. Restart Firefox to see the changes