--- modules/http/http_filters.c	(revision 1874808)
+++ modules/http/http_filters.c	(working copy)
@@ -1430,25 +1430,28 @@ 
     h.bb = b2;
 
     if (r->status == HTTP_NOT_MODIFIED) {
-        apr_table_do((int (*)(void *, const char *, const char *)) form_header_field,
-                     (void *) &h, r->headers_out,
-                     "Connection",
-                     "Keep-Alive",
-                     "ETag",
-                     "Content-Location",
-                     "Expires",
-                     "Cache-Control",
-                     "Vary",
-                     "Warning",
-                     "WWW-Authenticate",
-                     "Proxy-Authenticate",
-                     "Set-Cookie",
-                     "Set-Cookie2",
-                     NULL);
+      /*
+       * List of headers Gecko doesn't update on a 304 (or 206 partial content)
+       * https://searchfox.org/mozilla-central/source/netwerk/protocol/http/nsHttpResponseHead.cpp#862
+       */
+        apr_table_unset(r->headers_out, "Connection");
+        apr_table_unset(r->headers_out, "Proxy_Connection");
+        apr_table_unset(r->headers_out, "Keep_Alive");
+        apr_table_unset(r->headers_out, "Proxy_Authenticate");
+        apr_table_unset(r->headers_out, "Proxy_Authorization");
+        apr_table_unset(r->headers_out, "TE");
+        apr_table_unset(r->headers_out, "Trailer");
+        apr_table_unset(r->headers_out, "Transfer_Encoding");
+        apr_table_unset(r->headers_out, "Upgrade");
+        apr_table_unset(r->headers_out, "Content_Location");
+        apr_table_unset(r->headers_out, "Content_MD5");
+        apr_table_unset(r->headers_out, "ETag");
+        apr_table_unset(r->headers_out, "Content_Encoding");
+        apr_table_unset(r->headers_out, "Content_Range");
+        apr_table_unset(r->headers_out, "Content_Type");
+        apr_table_unset(r->headers_out, "Content_Length");
     }
-    else {
-        send_all_header_fields(&h, r);
-    }
+    send_all_header_fields(&h, r);
 
     terminate_header(b2);