Should have a semicolon (added in red) but, that probably isn't it. It could be your browser settings, especially FF and NS settings.
Code:
<a href="http://domain.com/tour/master.htm" onclick="NewWindow(this.href,'Tour','550','400','no','center');return false;" onfocus="this.blur()"><img border="0" src="images/tour.gif" width="250" height="57"></a><p>
I would really need to see your page to have a shot at really being able to tell.
PLEASE: Include the URL to your problematic webpage that you want help with.
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