The connection is not secure (HSTS) / Firefox Solved

Question

Hello,

Since yesterday (08/05/2016), I'm receiving this message on Firefox:
http://img4.hostingpics.net/pics/883777Capture.png

"The owners of www.youtube.com have incorrectly configured their website. To prevent your data from being stolen, Firefox has not connected to this website.
This site uses HTTP Strict Transport Security (HSTS) to instruct Firefox to only establish a secure connection. Thus, it is not possible to add an exception for this certificate."

"www.youtube.com uses an invalid security certificate. The certificate is not secure because the certificate of the issuing authority is unknown. The server may not be sending the appropriate intermediate certificates. It may be necessary to import an additional root certificate. Error code: SEC_ERROR_UNKNOWN_ISSUER"

This message appears on sites like Google, Firefox, Youtube, etc. but on others, it works perfectly. And all sites work on Chrome.

So I tried a number of things to fix this issue:
-Added the Microsoft Safety Family certificate
-Set network.stricttransportsecurity.preloadlist to false
-Deleted the cert8.db file
-Scanned my computer with Avira Antivirus and Malwarebyte AntiMalware, which are the only "antivirus" I use
-Checked that my clock and calendar were correct
-Uninstalled and reinstalled Firefox
-Cleared cookies and caches
-Applied practically all the solutions Firefox suggested for this issue

In short, I am a bit desperate since Firefox is my favorite browser and I am forced to use Chrome. I hope you can help me and thank you in advance, if you need any more info about the issue, feel free to ask!

Weima · Accepted Answer

My problem has been solved; I simply went to Options; Advanced; Network; I cleared the cache of Cached web content and Offline web content and user data.

Thileli · Answer

Hello,  Sometimes it's the proxy that blocks, so you need to go to options > advanced > network > settings and choose "Automatically detect proxy settings for this network."

saw2017 · Answer

In Kaspersky security products, you can disable the interception of secure connections:

Open the control panel of your Kaspersky software.
Click on Settings at the bottom left.
Click on Additional and then on Network.
If you are using a 2016 version of Kaspersky security products: in the Encrypted Connection Scan section, check the option Do not scan encrypted connections and confirm this change.
Otherwise, you can also click on Advanced Settings to try to trigger a reinstallation of the Kaspersky certificate. In the dialog window that opens, click on Install Certificate… and follow the on-screen instructions.
For the 2015 version, uncheck the option Scan encrypted connections.
Finally, restart your system for the changes to take effect.

saw2017 · Answer

The same error occurs on several secure sites

If you encounter this problem on multiple HTTPS sites that are not related, it means that your connection is being intercepted and receiving certificates rejected by Firefox. The interception often comes from software analyzing encrypted connections or from malware replacing the certificates of legitimate sites with their own.
Antivirus software

An antivirus software generally offers a feature for analyzing encrypted connections. Reinstalling it will restart the process of recognizing its certificates by Firefox. These steps apply to the following software:
Avast

You can disable the interception of secure connections:

Open the Avast dashboard.
Go to Settings > Active Protection and click on Customize next to Web Shield.
Uncheck Enable HTTPS scanning and confirm by clicking OK.

For more information about this feature, check out this page on the Avast blog.
Bitdefender

You can disable the interception of secure connections:

Open the Bitdefender dashboard.
For version 2016, click on Modules. For version 2015, click on Protection.
Click on Web Protection.
Disable SSL Scan.

For all Bitdefender products, refer to the Bitdefender support page.
Bullguard

With Bullguard security software, you can disable the interception of secure connections on certain specific sites (for example, major websites such as Google, Yahoo, and Facebook):

Open the dashboard of your Bullguard application.
Click on Antivirus Settings > Browsing.
Uncheck the Show safe results for websites that display this error message option.

ESET

With ESET security products, you can try disabling and re-enabling SSL/TLS protocol filtering or disabling the interception of secure connections, as described in ESET's support article.

The connection is not secure (HSTS) / Firefox

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