Convince Firefox to send an If-Modified-Since header over HTTPS - firefox

Question

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 http:// 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?
Here are some example HTTP and HTTPS request/response pairs, pulled using the Live HTTP Headers Firefox extension, with some differences in bold:
HTTP request/response:
http://myserver.com:30000/scripts/site.js
GET /scripts/site.js HTTP/1.1
Host: myserver.com:30000
User-Agent: Mozilla/5.0 (...) Gecko/2008070206 Firefox/3.0.1
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive
If-Modified-Since: Tue, 19 Aug 2008 15:57:30 GMT
If-None-Match: "a0501d1-300a-454d22526ae80"-gzip
Cache-Control: max-age=0
HTTP/1.x 304 Not Modified
Date: Tue, 19 Aug 2008 15:59:23 GMT
Server: Apache/2.2.8 (Unix) mod_ssl/2.2.8 OpenSSL/0.9.8
Connection: Keep-Alive
Keep-Alive: timeout=5, max=99
Etag: "a0501d1-300a-454d22526ae80"-gzip
HTTPS request/response:
https://myserver.com:30001/scripts/site.js
GET /scripts/site.js HTTP/1.1
Host: myserver.com:30001
User-Agent: Mozilla/5.0 (...) Gecko/2008070206 Firefox/3.0.1
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive
HTTP/1.x 200 OK
Date: Tue, 19 Aug 2008 16:00:14 GMT
Server: Apache/2.2.8 (Unix) mod_ssl/2.2.8 OpenSSL/0.9.8
Last-Modified: Tue, 19 Aug 2008 15:57:30 GMT
Etag: "a0501d1-300a-454d22526ae80"-gzip
Accept-Ranges: bytes
Content-Encoding: gzip
Content-Length: 3766
Keep-Alive: timeout=5, max=100
Connection: Keep-Alive
Content-Type: text/javascript
UPDATE: Setting browser.cache.disk_cache_ssl to true did the trick (which is odd because, as Nickolay points out, there's still the memory cache). Adding a "Cache-control: public" header to the response also worked. Thanks!

Answer 1

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.

Answer 2

HTTPS requests are not cached so sending an If-Modified-Since doesn't make any sense. The not caching is a security precaution.