first-child and last-child with IE8

If your table is only 2 columns across, you can easily reach the second td with the adjacent sibling selector, which IE8 does support along with :first-child:

.editor td:first-child
{
    width: 150px; 
}

.editor td:first-child + td input,
.editor td:first-child + td textarea
{
    width: 500px;
    padding: 3px 5px 5px 5px;
    border: 1px solid #CCC; 
}

Otherwise, you’ll have to use a JS selector library like jQuery, or manually add a class to the last td, as suggested by James Allardice.