Assuming your <a>
tag has no content, add a class to it tag with a position:relative; top:-50px;
Depending on your document, you my also have to wrap it in an absolute <div>
This should be cross-browser compatible if implemented correctly.
EDIT
This is the test I’ve done locally and it works fine in FF 3.6
<html>
<body style="margin:0; padding:0;">
<div style="position:fixed; height:50px; background-color:#F00; width:100%">
Fixed header
</div>
<div style="padding-top:50px">
<div style="height:100px; margin-top:10px; background-color:#0F0"><a href="#linkhere">Go to anchor</a></div>
<div style="height:100px; margin-top:10px; background-color:#0F0">Blah</div>
<div style="height:100px; margin-top:10px; background-color:#0F0">Blah</div>
<div style="height:100px; margin-top:10px; background-color:#0F0">
<a name="linkhere" style="position:relative; top:-75px;"></a>
Link here</div>
<div style="height:100px; margin-top:10px; background-color:#0F0">Blah</div>
<div style="height:100px; margin-top:10px; background-color:#0F0">Blah</div>
<div style="height:100px; margin-top:10px; background-color:#0F0">Blah</div>
<div style="height:100px; margin-top:10px; background-color:#0F0">Blah</div>
<div style="height:100px; margin-top:10px; background-color:#0F0">Blah</div>
<div style="height:100px; margin-top:10px; background-color:#0F0">Blah</div>
<div style="height:100px; margin-top:10px; background-color:#0F0">Blah</div>
<div style="height:100px; margin-top:10px; background-color:#0F0">Blah</div>
<div style="height:100px; margin-top:10px; background-color:#0F0">Blah</div>
<div style="height:100px; margin-top:10px; background-color:#0F0">Blah</div>
<div style="height:100px; margin-top:10px; background-color:#0F0">Blah</div>
<div style="height:100px; margin-top:10px; background-color:#0F0">Blah</div>
</div>
</body>
</html>