There is security software like Avast/AVG, Kaspersky, BitDefender and ESET that intercept secure connections and send their own certificate. If you are running any of the above software please check their community forum for a work around to apply settings.

• https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message#w_avast-and-avg-security-products

These pages are also for the errors you may have besides relating to the above.

• 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
• http://kb.mozillazine.org/Error_loading_websites
• https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER

Please let us know if this solved your issue or if need further assistance.

This issue can be caused by corrupted cookies or cookies that are blocked.

• check the permissions for the domain in the currently selected tab in "Tools -> Page Info -> Permissions"
• https://support.mozilla.org/en-US/kb/fix-login-issues-on-websites-require-passwords

Clear the Cache and remove the Cookies from websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Remove the Cookies" from websites that cause problems:

• Options/Preferences -> Privacy & Security
  Cookies and Site Data -> Manage Data

"Clear the Cache":

• Options/Preferences -> Privacy & Security
  Cookies and Site Data -> Clear Data -> Cached Web Content: Clear

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems