It works but the comment inside "showinbg" how to change the position is not working at all.
1. I have created the userChrome.css file at the c:\Users\USER NAME\AppData\Roaming\Mozilla\Firefox\Profiles\71b0e3u7.default-release\chrome folder.
2. Then added two lines:
/* New Tab Context - move the menu item to the top */
#tabContextMenu menuitem[label="New Tab"] { -moz-box-ordinal-group: 0 !important; }

3. Then restarted Firefox 74 and ... it had not changed the position to the first.

So, would you please change the comment to the working one?

Not working with FF 63.0b5 Beta....