Yes, Firefox normally doesn't trust an unknown "man in the middle" to issue website certificates. As you would hope!! Here are two workarounds to get Firefox to trust all of the fake certificates your proxy will generate: '''Option #1: Import the Signing Certificate''' If you import the Charles Proxy signing certificate into Firefox's certificate store, then all of its fake certificates will be trusted. (A) If you do not already have a certificate file ready to import, you can export it from IE or Chrome. * This may appear in IE's Certificates dialog OR it may appear when you view the certificate details on any secure page you load in IE/chrome * The Export or Copy to file button starts the Export Wizard. Use the DER format and save to a convenient location ''Example screenshots:'' https://support.mozilla.org/questions/1199797#answer-1064849 (B) When finished with all the necessary exports to complete the chain in the Certification Path, you can import the certificates into the Firefox Authorities tab: * Windows: "3-bar" menu button (or Tools menu) > Options * Mac: "3-bar" menu button (or Firefox menu) > Preferences * Linux: "3-bar" menu button (or Edit menu) > Preferences * Any system: type or paste '''about:preferences''' into the address bar and press Enter/Return to load it In the search box at the top of the page, type ''cert'' and Firefox should filter the list. Click "View Certificates" to open the Certificate Manager and click the "Authorities" tab. Then you can use the "Import" button to import the proxy server's certificate. (Fourth and fifth screenshots in the above-linked post.) ''When asked, I suggest allowing the certificate for websites only unless your IT suggests otherwise.'' It's a bit of pain, but the advantage of that approach is that you are making the minimal compromise of security. '''Option #2: Trust all Signing Certificates in the Windows Cert Store''' (A) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button accepting the risk. (B) In the search box above the list, type or paste '''enterp''' and pause while the list is filtered (C) Double-click the '''security.enterprise_roots.enabled''' preference to switch the value from false to true I'm not sure whether that will start working immediately or after the next time to exit Firefox and start it up again. I guess you'll know if you visit an HTTPS address and Firefox no longer objects. The disadvantage of this method is that any security compromise of the system certificate store will affect Firefox, too. This may be a lesser concern on a business system. Do either of those work for you?

On Mac, you would export from the keychain. E.g., https://support.mozilla.org/questions/1236194 Apologies for the oversight.

Thank you for your help! That solved it for me.