If you are not intending to upgrade to OS X 10.11 El Capitan for the moment, you should ensure that you download the Safari 9.0 update for OS X 10.9.5 or 10.10.5, as it contains many important security updates, as well as new features.
One major new feature is content-blocking, of course. Security fixes include:
- blocks user interface spoofing on malicious websites, via URL issues;
- a bug which could show LaunchServices’ quarantine history;
- compromised communications between Safari extensions and companion apps, and weak validation of extensions;
- failure to trigger warnings on some malicious websites;
- memory corruption which could have been exploitable;
- an attacker could create unintended cookies;
- leaks of browsing history, network activity, and more to crafted websites;
- cross-origin stylesheets could be loaded incorrectly;
- unauthorised HTTP requests could be sent.
These are important, and merit early updating.
It is available now in the Mac App Store.