Is it possible to use AppleScript to open a link in a new tab in Safari?
It's been a while since a new answer's been posted here. I think this is the optimal way to do this. It will open Safari if it's not open, create a new window if there are no windows open, and add the tab to the current (or newly created) window.
tell application "Safari"
activate
try
tell window 1 to set current tab to make new tab with properties {URL:theURL}
on error
open location theURL
end try
end tell