Firefox attempts to protect your computer from malicious websites by blocking access to sites that have expired or forged SSL (or Secure Socket Layer) certificates. SSL certificates help to verify the authenticity and ownership of a domain when attempting to establish a secure connection.

Firefox also blocks access to certain sites that have been reported to attempt installation of malicious software or those known to install exploits on user computers. If you want to browse a blocked website in Firefox, you must manually override the protection settings.

1) Remove Blocks for SSL Related Problems

1. Click the Firefox desktop icon to launch the Web browser on your computer. Alternatively, click "Start," "All Programs," "Mozilla Firefox" and then "Firefox."

2. Position the mouse cursor in the address bar and type "about:config" in the space. Click the green arrow at the end of the address bar or press the "Enter" key. Click the "I'll be careful, I promise!" button after the "This might void your warranty!" warning message about changing advanced settings appears on the screen.

3. Enter "browser.ssl_override_behavior" in the "Filter" text box. Locate the "browser.ssl_override_behavior" value in the "Preference Name" list and double-click it.

4. Delete the "2" value in the "Enter integer value" dialog box. Enter "1" in the dialog box and click the "OK" button.

5. Close the Firefox and restart it. Firefox no longer blocks access to sites with expired or unknown SSL certificates.

2) Access Other Untrusted Sites

1. Launch Firefox on your computer as described in Step 1 of Section 1. Navigate to a website that prompts Firefox to display the "This Connection is Untrusted" warning message.

2. Click the "Add Exception" button at the bottom of the warning message box.

3. Click and enable the "Permanently Store This Exception" option. Click the "Confirm Security Exception" button. The window closes and Firefox no longer displays the warning message or blocks access to the site. You must add the exception for every untrusted site you want to browse.

Read more: How to Disable "This Connection Is Untrusted" in Firefox