Since the release of OS X Lion, Safari when launched will restore any windows and tabs you may have had open when it was last quit.
Occasionally this can be a really useful feature but it can also be a laborious task to close each tab individually, especially if like me you tend to keep lots of tabs open. Here is a really simple tip that allows you to choose whether you want to restore tabs or not on an “ad hoc” basis:
When you want to open Safari without restoring tabs just hold down the shift key when launching the app from the dock.
This also works with the other Apple apps that take advantage of the OS X Auto Resume feature such as Text Edit.
Before you go
After spending over 20 years working with Macs, both old and new, theres a tool I think would be useful to every Mac owner who is experiencing performance issues.
CleanMyMac is highest rated all-round cleaning app for the Mac, it can quickly diagnose and solve a whole plethora of common (but sometimes tedious to fix) issues at the click of a button. It also just happens to resolve many of the issues covered in the speed up section of this site, so Download CleanMyMac to get your Mac back up to speed today.

thank you for your post! exactly what i was looking for right now
I got stuck on a site which wouldn’t let me close a box. I re-started 20 times, but couldn’t shake the old windows.
I tried starting with every key except the Shift key. Thanks!!!
Thank you for posting this tip…very helpful after I click on a site infect with a phishing scam that hijacked Safari.
Thank you! I also went to an infected site and was locked in. Your tip saves the day!