I wanted to set the width of the first column of the table, and I found this worked (in FF7) - the first column is 50px wide:
#MyTable>thead>tr>th:first-child { width:50px;}
where my markup was
<table id="MyTable">
<thead>
<tr>
<th scope="col">Col1</th>
<th scope="col">Col2</th>
</tr>
</thead>
<tbody>
...
</tbody>
</table>