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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
This issue is usually caused by corrupted cookies.
Clear the Cache and remove the Cookies for websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).
"Remove the Cookies" for 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
If clearing cookies didn't help then it is possible that the cookies.sqlite file in the Firefox profile folder that stores the cookies got corrupted.
- rename/remove cookies.sqlite (cookies.sqlite.old) and when present delete cookies.sqlite-shm and cookies.sqlite-wal in the Firefox profile folder with Firefox closed in case cookies.sqlite got corrupted.
You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.
- Help -> Troubleshooting Information -> Profile Folder/Directory:
Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder - https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data