Update: the websites seem to work if I try to keep refreshing it until the error does not show up. This is not supposed to happen since I can immediately access the website on my phone connected to the same network.

I also tried switching between using wireless connection and ethernet (on the same network) and it doesn't have any effect.

Other websites that do not work: https://www.qbittorrent.org/download https://play.basketball-gm.com/

Sounds like windows... What OS? What security software are you running? Are you running pro or home version of Windows? If pro, you should have Windows Sandbox enabled for testing. https://www.howtogeek.com/399290/how-to-use-windows-10s-new-sandbox-to-safely-test-apps/

Thank you for the response, jonzn4SUSE.

I am using Windows 11 Pro V10.0.26100 Build 26100. I tried Windows Sandbox and uninstalling Bitdefender but it didn't remove the error.

I found a fix after playing around with Firefox settings and turning off "Query OCSP responder servers to confirm the current validity of certificates" and then restarting the browser. Although I am not sure if I am making myself vulnerable to attacks.

Is this the correct and safe solution or is there something wrong with my OS? I've been using Firefox since 5 years ago and this is the first time that I have this kind of problem.

UPDATE: problem solved without turning off "Query OCSP responder servers..."

There is something wrong with the way Digital Root Certificates are installed on my Windows. It seems that Firefox is not the only app that is having a problem. Other apps that require Root Certificates (like the online game Galaxy Life) are also having the same problem with the same cause. Here's what I did (I did not know which specific step fixed my problem): - Open powershell as admin and run this command: certutil -generateSSTFromWU roots.sst - Download DigiCert Certificate Utility: https://www.digicert.com/support/tools/certificate-utility-for-windows - Go to https://www.digicert.com/kb/digicert-root-certificates.htm - Try to click on Demo sites links for each Root Certificate to check if that specific certificate is working properly on your computer. If there is a connection error on a specific demo link, try to install the corresponding certificate using DigiCert Certificate Utility. - Within DigiCert Certificate Utility, go to Tools > Windows Automatic Root Update and click "Auto Root Update"

I think there is also a way to manually install/update the certificates using Windows built-in tools but I was too lazy to read the convoluted instructions on the Windows forums.

Hmm, it's troubling that your system certificate stores might not be reliable.

Is your Firefox using the Windows certificate store? There is a preference to switch between the built-in store and the system store. Sometimes this is locked to the system store by an enterprise policy (set either by an IT department or by your security software). To investigate the current setting:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

(2) In the search box in the page, type or paste enterprise_roots and pause while the list is filtered - there should be two matching preferences

If security.enterprise_roots.enabled is set to true, then Firefox will trust certificates in the system certificate store. If it set to false (the classic setting back when users reported being annoyed with errors on servers that didn't send certs properly), then Firefox relies on its own certificate store.