I think usually this stores a cookie. If you want to clear cookies on a site -- please save any unsaved edits or other work first, and sign out of the site -- you can do it this way:

While viewing a page on the site, click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that. Firefox will ask you to confirm; go ahead and confirm.

Then try reloading the page (Reload button or Ctrl+R) and your device should be untrusted again. Does that work?

I've tried this approach. I'm still able to enter the site without 2FA. I've looked all over Firefox and can't find the info that's been loaded to avoid 2FA. Other browsers going to the same website still ask for 2FA so I know it's something within Firefox. This does not appear to be a simple solution. Even deleted cert9.db but that didn't help. Thanks for your interest. Maybe someone at Mozilla knows the answer.

Maybe the site uses multiple servers and we didn't clear cookies for the right server on the first round. If that isn't apparent during the login process, you can flush more cookies using the dialogs on the Settings page ("Clear Data" or "Manage Data").

See: Clear cookies and site data in Firefox

I went into settings and selected history. Checked all the boxes in clear recent history and the menu shows 0 bytes of site data. Thanks for trying, I've given up. they're hiding the reference very well.

I went into settings and selected history. Checked all the boxes in clear recent history and the menu shows 0 bytes of site data.

The "Clear Data" and "Manage Data" buttons are in the "Cookies and Site Data" section.