Safari appears to dislike having the return false occur in the function call. If you move it into your onsubmit as onsubmit="openPop(this.action);return false;" then Safari will work without issue.
Edit: To improve the answer, onsubmit itself needs to return false, so openPop returning false is not enough. You could just have it do onsubmit="return openPop(this.action);" though.
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