Is this about resetting Firefox or do you want to disable access completely?

You can consider to use code in the userContent.css file. Note that userContent.css and userChrome.css are disabled when Firefox in started in Safe Mode (e.g. hold Shift key when starting).

• http://kb.mozillazine.org/userContent.css

@-moz-document url(about:support){body {display:none !important; }}

The customization files userChrome.css (user interface) and userContent.css (websites) are located in the chrome folder in the Firefox profile folder.

• http://kb.mozillazine.org/Editing_configuration

Where do I find the userContent.css file? It's not there.

You need to create the userContent.css file and possibly the chrome folder as well if they do not exist yet. Note that the names are case sensitive.

• Create the chrome folder (lowercase) in the <xxxxxxxx>.default profile folder if this folder doesn't exist
• Use a plain text editor like Notepad to create a (new) userContent.css file in the chrome folder (file name is case sensitive)
• Paste the code in the userContent.css file in the editor window
• Make sure that you select "All files" and not "Text files" when you save the file via "Save file as" in the text editor as userContent.css.
  Otherwise Windows may add a hidden .txt file extension and you end up with a not working userContent.css.txt file