Firefox Error: “Software is Preventing Firefox From Safely Connecting to This Site”

This article explains how to fix a specific error that prevents Firefox from displaying pages due to Sophos preventing the connection.

 Issue

When opening Firefox, the following error message is displayed:

 Resolution

1. Open Firefox.

2. In the address bar, type about:config and hit Enter.

3. When prompted to “Accept the Risk and Continue”, do so.

4. In the preferences bar, type Security.pki.mitm_canary_issuer and hit Enter.

5. Click the pencil icon next to the entry. Delete the whole line.

6. Quit and relaunch Firefox. Load a webpage to confirm the error is gone.