None of that speaks to HSTS/Pinning... which is the feature meant to protect against this sort of thing. I'm specifically asking about how a company can bypass HSTS/Pinning without modifying my local browser.
Everything I'm reading indicates that's not possible.
>Firefox (and Chrome) disable Pin Validation for Pinned Hosts whose validated certificate chain terminates at a user-defined trust anchor (rather than a built-in trust anchor). This means that for users who imported custom root certificates all pinning violations are ignored.
That last sentence is key. From Wikipedia: some browsers "disable pinning for certificate chains with private root certificates to enable various corporate content inspection scanners and web debugging tools. The RFC 7469 standard also recommends disabling pinning violation reports for such certificate chains."
(all this for Windows, I believe the same is true for OS X, Linux depends on your specific your setup)
Internet Properties -> Content -> Certificates -> Advanced
http://security.stackexchange.com/a/2920
(Note: MITM is just one way companies monitor employees, but by no means the only way. If your company provided your work computer to you, or if they installed anything on your BYOD computer, I would treat everything you do on that computer as cc'ed to your boss by default.)