I recently moved away from Mac OS Sierra (yes I know I'm late) to High Sierra and I was forced to update to Safari Version 13.1.2. Now I am profoundly annoyed by it opening new Tabs (⌘t) next to my active tab instead of all to the right as it used to.
Knowing now that there is a menu entry "New Tab at End", there is no need for an Apple script or QuickSilver or anything. Just open System Preferences > Keyboard > Shortcuts, and add a shortcut using the plus button like the following: