With the new 64bits versions of Firefox Quantum V58 and higher tabs are no more below the address line and favorites bar, Mozilla will no fix it, the only solution is the css file that I will depict below:
1- Click Open Menu
2- Click Help
3- Click Troubleshooting Information
4- Click Open Folder
5- Click New Item/Folder
6- Name the folder as chrome
7- Click New Item/New Document
8- Name file as userChrome.css
9- Put this data in the file:
For best results, show Title Bar or Menu Bar */
#nav-bar { /* main toolbar */
-moz-box-ordinal-group: 1 !important;
border-top-width: 0 !important;
}
#PersonalToolbar { /* bookmarks toolbar */
-moz-box-ordinal-group: 2 !important;
}
#TabsToolbar { /* tab bar */
-moz-box-ordinal-group: 3 !important;
}
10- Restart Firefox, Enjoy.