As long as the browser itself is unmodified, you can detect MITM by checking the certificate chain by clicking on the padlock icon next to the URL bar. When the connection isn't being MITMed, the root of the certificate chain should be a public CA. All public CA publishes their public key hash or you can cross check the root's hash against the root store you download directly from Mozilla. If you are really paranoid, you want to download the root store from a connection that you know for sure is secure.
In practice, most Enterprise interception policy aren't meant to be hidden from their users/employees, so the CN field of the root certificate usually would clearly indicates that the root certificate belongs to the enterprise rather than one of the Public CA. Therefore, a much simpler way to detect MITM in this case is to simply check the name in the "Verified by:" field in the padlock icon popup. This is slightly less robust though, because it depends on the enterprise not deliberately going out of their way to try to hide that they're intercepting.
If the browser is modified, then all bets are off as the browser can lie to you. Note that installing a classic (non-Webextension) add-on counts as modifying the browser.