Is it possible to use AppleScript to open a link in a new tab in Safari?
I think this also does what you asked for, but it is much shorter and is less browser-specific:
do shell script "open http://www.webpagehere.com"
This will open the specified URL in your default browser. And if you explicitly want to open it in Safari, use this:
do shell script "open -a Safari 'http://www.webpagehere.com'"