Version 3.1.1 (5525.20) Firefox can do this, give users the ability to check a box so links that would normally open a new window (target="_blank") would open in a new tab instead.
Tabs are part of the Safari browser, not of the WebKit framework. Enhancement requests for Safari should be filed at <http://bugreport.apple.com/>. See <http://webkit.org/blog/101/back-to-basics/> for more details.