You can generate your <a> as text:
<xsl:text disable-output-escaping="yes"><a href="</xsl:text>
<xsl:value-of select="concat('http://myserver/_forms?url=', @FileRef, '&id=5')" disable-output-escaping="yes" />
<xsl:text disable-output-escaping="yes">" >/a<</xsl:text>
I know I'm a bit late on this, but I think the attribute tag is the way to, you just don't want to concat...
<a>
<xsl:attribute name="href">
http://myserver/_forms?url=<xsl:value-of select="@FileRef" disable-output-escaping="yes" />&id=5
</xsl:attribute>
</a>