You can block specific domains using the WebsiteFilter policy like this:

{
  "policies": {
    "WebsiteFilter": {
      "Block": ["*://*.ihatetheseguys.net/*"]
    }
  }
}

I outlined this in a lot more detail on this previous thread. You can also refer to Customize Firefox using policies.json or Customize Firefox using Group Policy (Windows) for more general steps.

Alternatively, you may want to find a website blocking add-on instead.

Hope this helps.

Hey Wesley, those sound like great solutions. It would be so much better to control things by the browser itself instead of relying on fickle extensions. Thank you very much.

It sounds like the WebsiteFilter policy is what you want then, since it's programmed into Firefox and doesn't require any extensions.

Upon rereading this it appears that this is for typed in URL's. I need something that will prevent ads from specific URL's. Isn't that different? BTW, you mentioned this site:

https://github.com/mozilla/policy-templates/blob/master/README.md#websitefilter

and on this site it mentions

Software\Policies\Mozilla\Firefox\WebsiteFilters\Block\1 = "<all_urls>"

Software\Policies\Mozilla\Firefox\WebsiteFilters\Exceptions\1 = "http://example.org/*"

I could not find any folder called Policies on C drive on my updated Win7 build. Or is this for Win10?

This policy applies to any attempt to load the website, not just ones that you manually visit.

You are looking for the C:\Windows\PolicyDefinitions folder. If you have that on your computer, you can put these two ADMX files there.

If not, you will need to create a policies.json file and then use the JSON code to apply this blocker.

Hope this helps.