what is the most updated safari I can use on my 2009 MacBook. I have OS X El Captain version 10.11.6?

Safari 11.1.2 is part of a security patch, not the OS, and OFN73 doesn't say what version they're using.

Even if their computer can't be upgraded, there isn't enough information to tell if they have the latest compatible one.

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Since Safari is installed as part of the operating system, it can only be upgraded if the OS is able to be upgraded. So, unless you can install a newer OS, you have the latest version of Safari.

If you mean MacBook Pro, or a MacBook model incapable of High Sierra, 11.1.2. If that's not enough, use a different web browser when needed.

If your computer can run High Sierra, 13.1.2.

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