Assuming you are already using deep inspection on Fortigate, you would first need to remove the stored HSTS flag on Firefox. Follow the instructions below and advise results:
Open your current Firefox settings (AKA Firefox profile) folder using either
[ul]"3-bar" menu button > "?" button > Troubleshooting Information(menu bar) Help > Troubleshooting Informationtype or paste about:support in the address bar and press Enter[/ul]
In the first table on the page, click the "Open Directory" (or similar) button. This should launch a new window listing various files and folders in your file browser.
Leaving that window open, switch back to Firefox and Exit/Quit, either:[ul]"3-bar" menu button > "power" button(menu bar) File > Exit / Quit[/ul]
Pause while Firefox finishes its cleanup, then open SiteSecurityServiceState.txt in your preferred text editor and delete all lines for the hostname you need to access and save the file.
When you start Firefox again, on your first visit, Firefox normally ignores the HSTS status because it hasn't gotten past the handshake.
NSE5, CCSE, CCNA R&S, CompTIA A+, CompTIA Network+, CompTIA Security+, MTA Security, ITIL v3