说服Firefox通过HTTPS发送If-Modified-Since标头

How can I convince Firefox (3.0.1, if it matters) to send an If-Modified-Since header in an HTTPS request? It sends the header if the request uses plain HTTP and my server dutifully honors it. But when I request the same resource from the same server using HTTPS instead (i.e., simply changing the https:// in the URL to https://) then Firefox does not send an If-Modified-Since header at all. Is this behavior mandated by the SSL spec or something?How can I convince Firefox (3.0.1, if it matter