Updated to work with a change to MetaFilter's HTML. Used to use

divToHighlight.lastChild.innerHTML += '<span class=\'tehhundUserScriptCommentCount\'>Comment ' + (i+1) + ' of ' + allCommentDivs.length + '. </span>';

now uses

divToHighlight.querySelector('.smallcopy').innerHTML += '<span class=\'tehhundUserScriptCommentCount\'>Comment ' + (i+1) + ' of ' + allCommentDivs.length + '. </span>';