Hard to tell without the error message. Using Nightly builds or betas can always be problematic.
Here is the message shown when it's not possible to get to the web sites. This particular site was google.com.
Warning: Potential Security Risk Ahead
Nightly detected a potential security threat and did not continue to
www.google.com. If you visit this site, attackers could try to steal information like your passwords, emails, or credit card details.
What can you do about it?
www.google.com has a security policy called HTTP Strict Transport Security (HSTS), which means that Nightly can only connect to it securely. You can’t add an exception to visit this site.
The issue is most likely with the website, and there is nothing you can do to resolve it.
If you are on a corporate network or using anti-virus software, you can reach out to the support teams for assistance. You can also notify the website’s administrator about the problem.
Learn more…
Report errors like this to help Mozilla identify and block malicious sites