Intermediate CA Preloading
In pre-release versions of Firefox users are benefiting from a new feature called Intermediate CA Preloading which reduces the number of secure connection errors that users run into when web servers forget to properly configure their TLS response.
In Bug 1404934 we've added support to download the Intermediate Certificate Authorities that have been disclosed to the Mozilla CA Root Program from Kinto in the background during normal Firefox operation.
This is currently only enabled for Nightly and Beta users on desktop platforms. (January 2020)
Remaining work is being tracked by Bug 1535662.
What it does
Websites using encryption should provide two digital PKI certificates  when connecting to clients: One for the website itself, and one for the intermediate CA that produced the website's digital certificate. Sometimes, websites are set up incorrectly.
When other browsers encounter this case, they use a mechanism where the user's browser then, in the background, connects to the CA and downloads the certificate just-in-time, which delays the connection and reveals the website the user is connecting to.
Preloading the intermediate CA data avoids the just-in-time network fetch, which delays the connection.
Avoiding the network fetch is also a privacy improvement, as it doesn't disclose your browsing pattern to the certificate authority that issued the misconfigured website's certificate.
It's also helpful because it's possible to "fingerprint" a user by a website carefully analyzing what other websites load or do not load due to which intermediate CAs have been "seen" by a user. Preloading ensures that all users have "seen" all the same intermediate CAs.
The Mozilla CA program requires all members to disclose intermediate CAs before using them, as a safety measure. However, there is not currently a way to technically enforce that. Intermediate Preloading can eventually perform that enforcement, by only trusting intermediate CAs which were disclosed already. This protects against a scenario where a compromised CA is coerced into producing a secret intermediate CA, which is then used to attack people on the Internet.
How it works
Intermediate Preloading fetches ~100 intermediate certificate authorities' certificates once a day during the Kinto main update , and loads them into your profile, as if you had visited a site that used that intermediate. At 100 per day, summing to between 300-500 kB, it will take approximately three weeks for a Firefox profile to preload all intermediates . We will likely increase the rate after the initial experiments.
The certificate data is loaded into the NSS Certificate Database, as is done for normal web browsing. In the future, we will use the faster Rust "rkv" database, in Bug 1530545.
Currently preloading is only enabled for desktop users on Nightly. We will want to restrict the download to be only over WiFi before enabling on mobile.
Intermediate Preloading should reduce the number of SEC_ERROR_UNKNOWN_ISSUER errors seen by Firefox users over time, which is our most common error.
Telemetry for Intermediate Preloading is available in the histograms:
and the scalars:
 The WebPKI generally has one root CA certificate, one intermediate CA certificate, and then one end-entity (specific website) certificate. Sometimes there can be more than one intermediate CA certificate, potentially much more than one. (https://tls-observatory.services.mozilla.com/static/certsplainer.html?id=188088012)
 100/day is configurable by a pref, it is likely to change. See https://searchfox.org/mozilla-central/source/security/manager/ssl/security-prefs.js#166 .
 The data is loaded from Kinto here: https://settings.prod.mozaws.net/v1/buckets/security-state/collections/intermediates/records . This data is exported from the Common CA Database maintained by the Mozilla root program.