The displayed error may be shown when your certificate setup is out of date - if you’re using Windows 7, you have to renew them manually (since Microsoft doesn’t support Windows 7 anymore). A lot of websites rely on those certificates for authentication purposes. Please check whether the DST Root CA X3 certificate is still valid on your computer, and if you have the new ISRG Root X1 already installed by any chance. You can do so by running certmgr.msc in the command line (press the Windows key + R to bring up the Run command, and type certmgr.msc and press Enter).
If you don’t have the ISRG Root X1 certificate set up yet, here’s a quick-how to:
Download the ISRG Root X1.der certificate file from https://letsencrypt.org/certificates/
Click the downloaded file to start the installation process (or left click and choose install).
Click install certificate.
Click “Next” on the next window.
Select “Place all certificates in the following store”.
Click “Browse” and select “Trusted Root Certification Authorities”. Confirm the changes by pressing “OK” and “Next”.
Finish the certificate installation by pressing “Finish”.
Run certmgr.msc in the command line to check if the installation was successful (the ISRG Root X1 Certificate should appear under the Trusted Root Certification Authorities Certificate Store).
ABOUT FIREFOX: Browsers other than Firefox generally use the operating system's facilities to decide which certificate authorities are trusted. Firefox instead had certificate list as part of the browser, but that has changed. Maybe you have an old version of Firefox. https://blog.mozilla.org/security/2021/07/28/making-client-certificates-available-by-default-in-firefox-90/