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Asked: 2026-05-10T13:38:38+00:00

How can I convince Firefox (3.0.1, if it matters) to send an If-Modified-Since header

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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!

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1 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.

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