Do you mean using Fiddler (great program, definitely a musthave) in place of Wireshark?
In that case you can add its key to a trusted authorities in Firefox and then it swaps nothing, everything seems to be signed properly... Unless I misunderstood your comment.
No, Fiddler is an active ManInTheMiddle attack. With HTTPS interception it always substitutes the certificate (which is the public key).
The substituted public key allows the proxy to negotiate a TLS session between the browser and the proxy, impersonating the real server.
You avoid a certificate error if you install the signing CA certificate in the browser; but you still tamper with the traffic. There are scenarios where mitm doesn't work; for example Client Authenticated TLS.
Things like certificate pinning, where the browser expects a specific public key, also break intetception.
I meant that Fiddler switches the original certificates with certificates that it generates. It's not a big deal if you trust them (on Windows Chrome and IE work automatically since it adds them to the trusted root store) and for Firefox you just have to trust the Fiddler issued certificate. However, if you inspect the certificate of an HTTPS site when Fiddler is running you see the CA is "DO_NOT_TRUST_FiddlerRoot".
I believe mitmproxy also sniffs HTTPS, but I think it uses a different method by dynamically generating a cert based on the true one http://mitmproxy.org/doc/howmitmproxy.html (bottom)
In that case you can add its key to a trusted authorities in Firefox and then it swaps nothing, everything seems to be signed properly... Unless I misunderstood your comment.