ASP .NET v4 multiline textbox firefox issue

We recently upgraded to .NET v4.0 to take advantage of the introduction of Page.MetaDescription and Page.MetaKeywords. Unfortunately, this appears to have caused a problem with multiline <asp:TextBox> controls in Firefox. The following code:
<asp:TextBox id="Description" TextMode="MultiLine" Columns="80" Rows="12" MaxLength="5000" runat="server" />
Generates the following HTML output:
<textarea name="ctl00$ContentPlaceHolder$Description" rows="12" cols="80" id="ctl00_ContentPlaceHolder_Description">
</textarea>
As you can see, rather than closing the textarea on the same line, the closing tag is generated on the next line down. Firefox displays this as a blank line in the first row of the textarea, as ca ...