Consider the following HTML:
Just to put all in one place.
The problem is produced when you have a cell with position relative inside a table with collapsed borders (as Boris indicated and filled in the bug https://bugzilla.mozilla.org/show_bug.cgi?id=688556)
This can be easily solved using CSS as indicated by user2342963 (Adding background-clip: padding-box to the cell).
You can see the problem (with Firefox) and the fix here: http://jsfiddle.net/ramiro_conductiva/XgeAS/
table {border-spacing: 0px;}
td {border: 1px solid blue; background-color: yellow; padding: 5px;}
td.cellRelative {position: relative;}
td.cellRelativeFix {background-clip: padding-box;}
table.tableSeparate {border-collapse: separate;}
table.tableCollapse {border-collapse: collapse;}
position: relative
position: static
position: relative
position: static
position: relative
position: static