When you say "blocked" do you get a message like access is forbidden, or a secure connection error, or a blank page, or a broken page?

In the absence of particular symptoms, I suggest trying "the usual":

If a site is generally known to work in Firefox, these are general suggestions to try when it stops working:

Cache and Cookies: When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

(1) Clear Firefox's Cache

See: How to clear the Firefox cache

If you have a large hard drive, this might take a few minutes. If you do not see the number going down on the page, you can reload it using Ctrl+r to check progress.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site, try either:

• right-click (on Mac Ctrl+click) a blank area of the page and choose View Page Info > Security > "View Cookies"
• (menu bar) Tools > Page Info > Security > "View Cookies"
• click the padlock or "i" icon in the address bar, then the ">" button, then More Information, and finally the "View Cookies" button

In the dialog that opens, the current site should be pre-filled in the search box at the top of the dialog so you can remove that site's cookies individually.

Then try reloading the page. Does that help?

Testing in Firefox's Safe Mode: In Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement? (More info: Diagnose Firefox issues using Troubleshoot Mode)

Here is the error I get. It occurs 'every' time I go to any site with https in the url. It seems though that after several tries, if I close the browser and reopen, it thereafter works okay. Here's the message:

An error occurred during a connection to www.pinstripealley.com. You have received an invalid certificate. Please contact the server administrator or email correspondent and give them the following information: Your certificate contains the same serial number as another certificate issued by the certificate authority. Please get a new certificate containing a unique serial number. Error code: SEC_ERROR_REUSED_ISSUER_AND_SERIAL

   The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.
   Please contact the website owners to inform them of this problem.

BTW, this problem only started yesterday. Never before.

You can check the date and time and time zone in the clock on your computer: (double) click the clock icon on the Windows Taskbar.

• https://support.mozilla.org/en-US/kb/what-does-your-connection-is-not-secure-mean
• https://support.mozilla.org/en-US/kb/troubleshoot-time-errors-secure-websites
• https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message

You can click the "Advanced" button to expand this section and show extra details. You can click the blue error message to expand this section. If the certificate is not trusted (SEC_ERROR_UNKNOWN_ISSUER) then click "Copy text to clipboard" and paste the base64 encoded certificate chain text in a reply. That will allow us to check details like the issuer of the certificate.

That shouldn't happen if you are connecting directly to a well-run site.

Do you use any of the security software discussed in this article: How to troubleshoot security error codes on secure websites?

Do you use any proxy server, private VPN service, peer-to-peer network (e.g., Hola), or onion routing (e.g., Tor) that could potentially modify certificates? You can check your Firefox connection setting here:

• Windows: "3-bar" menu button (or Tools menu) > Options
• Mac: "3-bar" menu button (or Firefox menu) > Preferences
• Linux: "3-bar" menu button (or Edit menu) > Preferences

In the left column, click Advanced. Then on the right side, with the "Network" mini-tab active, click the "Settings" button. The default of "Use system proxy settings" piggybacks on your Windows/IE "LAN" setting. "Auto-detect" can lead to a flaky connection. You may want to try "No proxy".

Thank you. Yes, definitely well-run sites. As I mentioned, it happens when I go to the NY Times.

I do use Kaspersky. I do not use any proxy server, private VPN service, peer-to-peer network (e.g., Hola), or onion routing.

When I did as you instructed, it says, "Use system proxy settings." So, that is where I stopped understanding what you were telling me. If it says "Use system proxy settings," should I leave it that way? Should I change it to "No proxy"? Is there a danger to doing that?

I don't understand why this started happening all of a sudden. Any other guidance would be appreciated.

BTW, what does this mean?

Your certificate contains the same serial number as another certificate issued by the certificate authority. Please get a new certificate containing a unique serial number.

Thanks!

Can you inspect the certificate?

You can click the "Advanced" button to expand this section and show extra details. You can click the blue error message to expand this section. If the certificate is not trusted (SEC_ERROR_UNKNOWN_ISSUER) then click "Copy text to clipboard" and paste the base64 encoded certificate chain text in a reply. That will allow us to check details like the issuer of the certificate.

You can open the Certificate Manager and go to the Servers tab. The Servers tab has an "Add Exception" to open the "Add Security Exception" window. You can type/paste the domain in the location field and click "Get Certificate" to retrieve the certificate and click the "View" button to inspect the certificate.

• Options/Preferences -> Advanced -> Certificates: View Certificates

Try to rename/remove the cert8.db file in the Firefox profile folder with Firefox closed.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Thank you. In what screen will I find the "Advanced" button and the "Copy Text to Clipboard" button.

This morning, when I checked my version of Firefox, it had me using v54 (64 bit). I know I had already updated to v55, so this is curious. I updated to v 55.0.2 (64 bit) and so far the problem hasn't arisen. I will observe for the next 24 hours or so. Perhaps that fixed the problem. Which would beg the question, "why was I running v 54 when I had already updated to v 55?"

Thanks. I'll keep you posted.

econoshamanic said

In what screen will I find the "Advanced" button and the "Copy Text to Clipboard" button.

For future reference, the button should appear on the error page pictured in this article: What do the security warning codes mean?

Some of the error codes in that section are links/buttons that show certificate information when you click them.