Those all sound like HTTPS sites. Have you noticed Firefox reporting any issues with the certificates presented by secure sites?

It has, and also with Google, but that loads fine. How does HTTPS factor in?

Well, it depends. If a site is blocked because Firefox doesn't trust the certificate and you create an exception, that exception only applies to the domain listed in the address bar. Many sites use separate servers for style sheets and those would still be blocked by a certificate error for that server, causing you to see unstyled pages.

You should never need to create exceptions for well-run sites. If you get errors for most secure sites, the problem usually is one of the following:

(1) Error in your system's date, time, or time zone, which throws off certificate validity checks. Sometimes allowing computers to use an internet-based time source can introduce this problem.

(2) Firefox not being set up to work with your security software that intercepts and filters secure connections. Products with this feature include Avast, BitDefender, Bullguard, ESET, and Kaspersky; AVG LinkScanner / SurfShield can cause this error on search sites.

(3) On Windows 10, Firefox not being set up to work with the parental control software Microsoft Family Safety. (To test by turning it off, see: http://windows.microsoft.com/en-us/wi.../turn-off-microsoft-family-settings)

(4) Malware on your system intercepting secure connections.

If you have any of those specific security products:

That would be the first thing to check. This support article will walk you through it: How to troubleshoot security error codes on secure websites.

If none of those ring a bell:

On a site where you have this problem, inspect the "Issued by" section of the certificate. To view that, you can use the Page Info dialog. Either:

• click the padlock or "i" icon in the address bar, then the ">" button, then More Information, and finally the "View Certificate" button
• right-click (on Mac Ctrl+click) a blank area of the page and choose View Page Info > Security > "View Certificate"
• (menu bar) Tools > Page Info > Security > "View Certificate"

I'm attaching an example for comparison showing the areas to check for clues about the certificate issue. (This is Yahoo mail, but the same areas would be relevant for YouTube, Reddit, Google, this site, etc.) Any pattern?

There is a pattern in the certificates: in the "Organizational Unit" section, it says "<Not Part of Certificate>" on all the sites I have mentioned, as well as every secure website that I can find in my history. I'm assuming this is what is causing my problem- is there a way to fix this?

I don't think a missing organizational unit in the "Issued to" section is important, that's sort of like a division of a company.

What about the "Issued by" section or "Certificate Hierarchy", are those diverse and showing no pattern?