Looks like it comes from here: https://email@example.com and it implements failover rules for system requests over malfunctioning proxies. The exact rules are laid out at the top of
api.jsif you extract that
* ProxyMonitor monitors system and protected requests for failures
* due to bad or unavailable proxy configurations.
* In a system with multiple layers of proxy configuration, if there is a
* failing proxy we try to remove just that confuration from the chain. However if
* we get too many failures, we'll make a direct connection the top "proxy".
* 1. Any proxied system request without a direct failover will have one added.
* 2. If a proxied system request fails, the proxy configuration in use will
* be disabled. On later requests, disabled proxies are removed from the proxy chain.
* Disabled proxy configurations remain disabled for 48 hours to allow any necessary
* requests to operate for a period of time. When disabled proxies are used as a
* failover to a direct request (step 3 or 4 below), the proxy can be detected
* as functional and be re-enabled despite not having reached the 48 hours.
* 3. If too many proxy configurations got disabled, we make a direct config first
* with failover to all other proxy configurations (essentially skipping step 2).
* This state remains for 48 hours before retrying without "direct".
* 4. If we've removed all proxies we make a direct config first and failover to
* the other proxy configurations, similar to step 3.
* 5. Starting with Fx92, we will only disable proxy configurations provided by
* extensions. Prior to 92, we could not definitively identify extensions from
* the proxyInfo instance.
* If we've disabled proxies, we continue to watch the requests for failures in
* "direct" connection mode. If we continue to fail with direct connections,
* we fall back to allowing proxies again.