image didn't upload with initial question

When I visit the home page, the main StackOverflow style sheet is loaded from this address:

https://cdn.sstatic.net/Sites/stackoverflow/all.css?v=7f9acf8065f0

If you click that link, what do you get? A normal load is a giant wall of plain text starting with:

html,body,div,span,applet,object,iframe,h1,h2,h3,h4,h5,h6,p,blockquote,

Do you get an error message that could shed light on why it isn't loading?

Thanks. That page was being blocked. Added an exception and all is good.

Since you mentioned ESET... there are numerous products that filter your connections by interposing themselves as a "man in the middle" and presenting Firefox with a fake certificate so they can decrypt your requests and the responses from sites. These include not only ESET / nod32, but also: Avast, BitDefender, Bullguard, Kaspersky, AVG LinkScanner / SurfShield, and the parental control software Microsoft Family Safety (Windows 8.1 and later). More info on those: How to troubleshoot security error codes on secure websites.

dsmith9_99 said

Added an exception and all is good.

Not necessarily good. Well run, legit sites should NEVER EVER need a secure certificate exception. Please investigate by checking who is the issuer of that certificate. You can do that using the Page Info dialog:

• 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"
• click the padlock or "i" icon in the address bar, then the ">" button, then More Information, and finally the "View Certificate" button

In the dialog that opens, the "Issued by" section is the key one to check. I've attached a screenshot for comparison. What do you find?