Is it just on one site or do you get that error on many sites? 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: #Open the Avast dashboard on the affected system. #Select Settings from the left sidebar menu. #Switch to Active Protection. #Click on Customize next to Web Shield. #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.