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 duti
HTTPS requests are not cached so sending an If-Modified-Since doesn't make any sense. The not caching is a security precaution.
The not caching on disk is a security pre-caution, but it seems it indeed affects the If-Modified-Since behavior (glancing over the code).
Try setting the Firefox preference (in about:config) browser.cache.disk_cache_ssl to true. If that helps, try sending Cache-Control: public header in your response.
UPDATE: Firefox behavior was changed for Gecko 2.0 (Firefox 4) -- HTTPS content is now cached.
HTTPS requests are not cached so sending an If-Modified-Since doesn't make any sense. The not caching is a security precaution.