求解DIV在FF下不能根据内容撑开

做了个类似于QQ空间的弹出DIV提示：代码如下
//    if (width == undefined) {
//        width = 180;
//    }
    if ($("tishiDIV"))
        document.body.removeChild($("tishiDIV")); //如果有创建过层,先删除再重建
    var divElement;
    divElement = document.createElement("div");
    divElement.id = "tishiDIV";
    divElement.setAttribute("display", "block");
    divElement.style.left = 0;
    divElement.style.top = 0;
    divElement.style.width = (document.all) ? document.body.scrollWidth : innerWidth;
    divElement.style.position = "fixed";
    divElement.setAttribute("z-index", "1");
    divElement.setAttribute("text-align", "center");
    var widthDiv = document.body.scrollWidth / 2 - 60;
    var top = document.body.clientHeight - (document.body.clientHeight / 2 + 50);
    var newDivHtml = "";
    newDivHtml += "<div id='infoDiv' style='position:absolute; z-index:9999;top:" + top + "px;left:" + widthDiv + "px;width:100%;text-align:left;'>";
    newDivHtml += "<div style='z-index:17; margin-top:10px;top: 26px; font-size:12px;width:100%'>";
    newDivHtml += "<table id=\"tableDiv\" border=\"0\" cellspacing=\"0\" cellpadding=\"0\"><tr><td width=\"50\" align=\"right\" valign=\"top\" class=\"left\">&nbsp;</td><td class=\"mid\">" + tishi + "</td><td width=\"12\" align=\"left\" valign=\"top\" class=\"right\">&nbsp;</td></tr></table>";
    newDivHtml += "    </div>";    newDivHtml += "</div>";    divElement.innerHTML += newDivHtml;    document.body.appendChild(divElement);    $("infoDiv").style.top = (document.all) ? document.body.scrollTop + 50 : pageYOffset + 50;
    setTimeout("$(\"infoDiv\").style.display='none'", 2000);css样式：/** 提示框样式 **/
.mid{ color:#ffffff; font-weight:bold; background:url(tan_02.gif); background-repeat:repeat-x; text-indent:3px; line-height:55px;}
.left{ width:50px; height:55px; background-image:url(tan_01.gif); background-repeat:no-repeat;}
.right{ width:12px; height:55px; background-image:url(tan_03.gif); background-repeat:no-repeat;}
关键代码这段：
<table id="tableDiv" border="0" cellspacing="0" cellpadding="0">
<tr>
<td width="50" align="right" valign="top" class="left">&nbsp;</td>
<td class="mid">fsfafaefafawfawfawfawfawf</td>
<td width="12" align="left" valign="top" class="right">&nbsp;</td></tr>
</table>在FF下<td class="mid">fsfafaefafawfawfawfawfawf</td>这个<td>因为没有指定宽度，所以不能自动撑开。但是如果直接复制代码到网页中又能撑开，用JS创建后就不撑开了，高手何解？