It's only a problem with Facebook? This ''could'' be caused by a "man in the middle." If you click the "Advanced" button on the error page to view more technical information, what code is shown there? They typically look like SEC_ERROR_SOME_PROBLEM. If Firefox says there is a problem with the certificate, you can share it with forum volunteers by clicking the code, then copying the coded certificate from the panel that opens below that. More info on troubleshooting security codes: [[How to troubleshoot security error codes on secure websites]]

It's working ok now. Don't know why.