GZipped files not being served for websites hosted on IIS 7

Question

I have enabled static compression for the websites in IIS, also I have the following settings in IIS applicationHost.Config file.

<httpCompression directory="%SystemDrive%\inetpub\temp\IIS Temporary Compressed Files">
        <scheme name="gzip" dll="%Windir%\system32\inetsrv\gzip.dll" />
        <staticTypes>
            <add mimeType="text/*" enabled="true" />
            <add mimeType="message/*" enabled="true" />                
            <add mimeType="*/*" enabled="false" />
        </staticTypes>
    </httpCompression>

All the static files are getting compressed and are being saved in %SystemDrive%\inetpub\temp\IIS Temporary Compressed Files, but they are not being used. I found this using YSLOW. Do I need to enable any other settings for the IIS to use compressed ZGIP files?

I am using Firefox version 3.6.8 and the request header has Accept-Encoding: gzip,deflate

Can I force IIS to use compressed files?