Check out why the site is untrusted (see the Technical details) and if this is caused by a missing intermediate certificate then see if you can install this intermediate certificate from another source.

You can retrieve the certificate and check details like who issued certificates and expiration dates of certificates.

• Click the link at the bottom of the error page: "I Understand the Risks"

Let Firefox retrieve the certificate: "Add Exception" -> "Get Certificate".

• Click the "View..." button and inspect the certificate and check who is the issuer.

You can see more Details like intermediate certificates that are used in the Details pane.

Some firewalls monitor secure (https) connections and send their own certificate instead of the website's certificate.
Some examples are ESET and Bitdefender.

• ESET setup -> advanced setup -> extend web and email tree -> SSL
• SSL protocol: Do not scan SSL protocol

• BitDefender -> Privacy settings -> disable Scan SSL

With Gmail, the Technical Details say the following:

    accounts.google.com uses an invalid security certificate.

    The certificate is not trusted because no issuer chain was provided.

    (Error code: sec_error_unknown_issuer)

There is no option to bypass this message. Also, I use ESET and have already checked to make sure that the option is set to not scan SSL protocol. And this still does not tell me why Facebook and other websites that I have allowed the exception for do not display images.

This whole thing is very frustrating to me since Firefox worked perfectly prior to it being updated to version 18. And I can access all of my sites from within IE9 and Google Chrome.

Do you have any other ideas?

Did you check the certificate like I posted above?

For SSL communication to work properly in your browsers/email clients, it is essential that the root certificate for ESET, spol. s r.o. be added to the list of known root certificates (publishers). Therefore, the Add the root certificate to known browsers option should be enabled. Select this option to automatically add the ESET root certificate to the known browsers (e.g. Opera, Firefox). For browsers using the system certification store, the certificate is added automatically (e.g. Internet Explorer). To apply the certificate to unsupported browsers, click View Certificate > Details > Copy to File... and then manually import it into the browser.

(source: page 64 from http://download.eset.com/manuals/eset_ess_5_userguide_enu.pdf)

in case you cannot solve it that way you could also contact eset's support...

@cor-el: There is no link at the bottom of the page that says "I understand the risks". There's just "Get me out of here" and "Technical Details". I've already done that method with several other websites and added exceptions in, so I don't know what to do.

@madperson: The option you indicated is grayed out if I have the "Do not scan SSL protocol" option enabled. It is checked, though. Also, I seriously doubt that it's my NOD32 that's causing the problem for two reasons. First, Firefox only started giving me grief after it updated to the latest version. Second, I temporarily disabled the security software and it didn't make a difference.

Make sure that you do not run Firefox in (permanent) Private Browsing mode.

• https://support.mozilla.org/kb/Private+Browsing

• Tools > Options > Privacy: Use custom settings for history
• Deselect: [ ] "Always use private browsing mode"

Also check via the right-click context menu that you aren't opening pages in a frame.

I've never run Firefox in Private Browsing mode. I'm not sure what you mean by opening pages in a frame, but I always open up a new tab and type in a url directly to the bar.

Well that's weird. I downgraded to Firefox 13 (it was the only one I actually had) and restarted my computer. And that seemed to have fixed it. So, thanks.