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I'm far from a security expert, so the premise of my question might be wrong. If so please let me know.
In an enterprise setting, browser users are vulnerable to, and often subjected to, MITM attacks by the enterprise. The enterprise has complete control over how the browser is configured, so it simply pre-installs its own CA root certificate and configures the browser to trust it. The enterprise can then issue certificates for any website on the internet, and read or modify HTTPS traffic to them. For example, if a user accesses their online bank, they have no way of knowing if a rogue employee in the IT dept is intercepting their online banking password. Worse, they will be incorrectly reassured by all the expected security indicators (green padlock, correct name on the certificate, etc).
Revoking trust in the enterprise's root certificate would solve this, but the enterprise can prevent such configuration changes, and doing so would prevent the browser accessing intranet sites anyway, so it's not a solution.
Finally, since the enterprise (attacker) has complete control over the machine, it could just change the browser binary to display false security indicators (or install keyloggers, screen grabbers, etc). So let's assume that the enterprise's motivation only extends as far as configuring the browser and monitoring the network, and that the browser binary is unmodified. Let's also assume the browser is Firefox, since that's what I use.
Is it possible for a NOVICE user, under these conditions, to reliably detect MITM attacks by the enterprise?
If not, could changes made outside the control of the enterprise -- to Firefox itself, or to web security infrastructure -- block the enterprise from making a MITM attack without the user knowing?