This new way of handling tabs is horrible. The + icon to add a new tab is alway at the far right. Huge tabs just don't make any kind of sense to me. Complaints about this new feature are everywhere online. I'm guessing that Apple will change their mind soon and at least give the option to restore tabs to their previous behavior.
In the meantime, I've been simulating the old behavior by immediately opening five or six new empty tabs as soon as I open a new Safari window.
As soon as Apple decides to change this silly tab behavior or if someone else finds a better work around, I'll let you know.