Even if you don't plan to upgrade to Mountain Lion, as of today Safari 6 is available as an update for older versions of OS This new version includes numerous security fixes and improvements. 

For a (long) list of fixed bugs, the the Apple security announcement [1]. There are also some new security related features:

• extensions can now figure out if you are in private browsing mode, which should make it easier for extensions to avoid leak.
• the "https" in https urls is highlighted more, and the lock with more information about the certificate is placed right next to it.
• The safe password feature got redone, but it doesn't look like Safari will suggest new passwords unless you run Mountain Lion.

[1] http://support.apple.com/kb/HT5400

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Johannes B. Ullrich, Ph.D.
SANS Technology Institute
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