Perform a System File Check to fix corrupted system files Win 7, 8, Vista: https://support.microsoft.com/en-us/kb/929833

See if there are updates for your graphics drivers https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

This did not fix it. This is what I get when trying to log in.

An error occurred during a connection to www.bing.com. Peer's certificate has an invalid signature. (Error code: sec_error_bad_signature)

   The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.

http://www.ehow.com/how_11385212_troubleshoot-reset-connection-firefox.html

https://support.mozilla.org/en-US/kb/server-not-found-connection-problem

https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can

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

This Connection is Untrusted is sometimes caused because the computer system clock is wrong. Check the time / date / time zone settings.

The most common reason for getting "sec_error_bad_signature" on numerous sites seems to be the Avast Web Shield. Web Shield intercepts your browsing and filters it, but to filter secure sites, it presents "fake" certificates to Firefox. Avast should have set up Firefox to trust its fake certificates but that seems to fail on a regular basis.

To test this theory, try turning off scanning of encrypted sites. I saw these steps in another post:

1. Open the Avast dashboard on the affected system.
2. Select Settings from the left sidebar menu.
3. Switch to Active Protection.
4. Click on Customize next to Web Shield.
5. Uncheck the "Enable HTTPS Scanning" option and click ok

If that resolves the issue, but you prefer to filter encrypted communications, you may need to import the Avast! signing certficate into Firefox's Certificate Manager, Authorities tab.