Why is Firefox (and only Firefox) reporting that my connection is insecure on multiple sites?

There is a lot to unpack so I’ll do my best here (based on some assumptions).

  1. Firefox maintains its own certificate store which is likely the reason only Firefox is throwing these errors. Traditionally, SysAdmins will push out certificates through Group Policy, which works for both Chrome and IE / Edge but Firefox won’t trust it. I would imagine that your traffic is being intercepted by a transparent proxy server which is inspecting your traffic (note that looking at the certificate information will reveal whether or not this is a certificate that your work has pushed out).

  2. Assuming again, but your work is probably explicitly not filtering financial website traffic — presumably to avoid any potential liability with doing so.

  3. I have no idea why some load as plain text. This might be something to do with the proxying process.

    EDIT: As Arminius astutely pointed out, pages loading as plain text is likely due to certificate errors happening with resources being pulled from third party domains. It is likely that the images and CSS are not loading as the cert errors from those domains prevent the resources from being transmitted.

Why is Firefox the only browser reporting these security errors?

As already said, Firefox use its own Certificate Authority store, managing it from a company point of view is hard and usually not worth it when Chrome is allowed.

Why isn't Firefox reporting security errors on banking and financial websites?

HTTPS inspection by a transparent proxy is usually not allowed by laws as it would break the banking confidentiality of the user and is generally considered as illegal.

Why do some pages not report security errors, but only load as plain text?

Usually (from what saw) it is because the front page is categorized as 'should not be intercepted' as financial sites are (which is the case for Wells Fargo on bluecoat's list here), but the images come from another CDN, so the interception fires on the CDN and the images are not loaded because the authority certificate is unknown to Firefox.

In my case, Avast was interfering with proper site loading on Firefox but it was fine on other browsers.

I un-ticked "scan safe connections" in the Web Shield settings and my problems were solved.