Hi Alex, apologies about the reload. Users sometimes report that they get an older "Share Data" experience than the one we've had for the past couple of years, and apparently they haven't tracked that down yet.

So currently, Firefox doesn't have a feature to reject external links. This is controlled by the Windows setting for how to open the HTTP and HTTPS protocols. In theory, you could assign these to Notepad (or another program) so that programs seeking to launch a URL in your browser instead call up Notepad (or the other program). That wouldn't be useful if you ever want to really open a link, but it would be resolve the immediate frustration.

I wasn't able to do this quickly on Windows 10 through the Default Apps control panel. I think it might require a registry edit nowadays??

So currently, Firefox doesn't have a feature to reject external links.

Maybe I'm naive, but that seems like a security risk. What if a program was infected, and opened up a site with illegal material or viruses on it?

I'm sure that does happen, but malware authors often use an invisible browser so you don't even know what they're up to.

To protect against bad websites, you can keep Firefox's phishing and malware protection enabled. But since that doesn't update instantly when new problem sites are discovered, you could layer on additional protection with security software that intercepts your browsing requests and/or reputation services like OpenDNS Home (basic anti-phishing is free, https://www.opendns.com/home-internet-security/#benefit-matrix).