Hi savenko.67, glad to see you on Mozilla Support Forum.

• To do this, click the menu button at the top right corner of the Firefox window.

• Click the Options option near the bottom of the menu.

• Click the General menu in the Options tab.

• You will find Network Settings at the bottom of the page.

• Click on Settings.

• Choose No Proxy.

If it doesn't solved your problem, then feel free to ask in Reply Section. Thanks for raising your question in Mozilla Support Forum.

There is security software like Avast, Kaspersky, BitDefender and ESET that intercept secure connection certificates and send their own.

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

https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message

https://support.mozilla.org/en-US/kb/connection-untrusted-error-message

Websites don't load - troubleshoot and fix error messages

http://kb.mozillazine.org/Error_loading_websites

What do the security warning codes mean

Thanks for the help! But none of this helped. Solved the problem simply; cleaned the cookies and everything is now in order. And the antivirus is installed and did not identify the problem.

That was very good work. Well Done.
Please flag your last post as Solved Problem as this can help others with similar problems.