You can check the date and time and timezone 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/secure-connection-failed-firefox-did-not-connect
• https://support.mozilla.org/en-US/kb/troubleshoot-time-errors-secure-websites

Date, time and time zone are set to update automatically. The issue arose a few days after the clocks went back an hour.

Did you make sure that the clock is having the correct time? You can possibly try a different internet Time service.

Are you using a proxy or VPN?

You can check the connection settings.

• Settings -> General -> Network: Connection -> Settings

If you do not need to use a proxy to connect to internet, select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly.

See "Firefox connection settings":

• https://support.mozilla.org/en-US/kb/websites-dont-load-troubleshoot-and-fix-errors
• https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can
• https://support.mozilla.org/en-US/kb/firefox-and-other-browsers-cant-load-websites

Correct time is displayed, not using proxy or vpn.

Does it mention a reason on the error page or show an Advanced button you can click for more detail ?

Can you attach a screenshot?

• https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem
• use a compressed image type like PNG or JPG to save the screenshot

It says the authenticity of the received data could not be verified.

Better resolution screenshot

The screenshot shows the PR_CONNECT_RESET_ERROR message.

• https://support.mozilla.org/en-US/search/?product=firefox&w=2&q=PR_CONNECT_RESET_ERROR

This means that somewhere along the connection the connection got closed.

What security software do you have in case that software is interfering ?

You can check the connection settings.

• Settings -> General -> Network: Connection -> Settings

If you do not need to use a proxy to connect to internet, select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly.

See "Firefox connection settings":

• https://support.mozilla.org/en-US/kb/websites-dont-load-troubleshoot-and-fix-errors
• https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can
• https://support.mozilla.org/en-US/kb/firefox-and-other-browsers-cant-load-websites

Already set to no proxy. security wise I've got windows defender and Kaspersky total security, turning Kaspersky off makes no difference.

Boot the computer in Windows Safe Mode with network support to see if that has effect.

• https://www.bleepingcomputer.com/tuto.../how-to-start-windows-in-safe-mode/
• https://www.digitalcitizen.life/4-ways-boot-safe-mode-windows-10/