This post explains how to fix the SEC_ERROR_INADEQUATE_KEY_USAGE error that sometimes occurs when you visit a secure url in Firefox.

The Problem

When loading a secure url (https) in Firefox, you see "Secure Connection Failed", "Error code: SEC_ERROR_INADEQUATE_KEY_USAGE".

The Solution

  1. Go to about:profiles in Firefox. Do this by entering "about:profiles" in the address bar like a URL.
  2. Locate the profile that is currently in use. This will be the profile that has no Remove button.
  3. Click Open Folder in Root Directory.
  4. Delete the cert9.db and cert_override.txt files. You may need to close Firefox to delete these files.
  5. Try problematic url again. The problem should be resolved.