Is this problem on your Android device, or on Windows or MacOS? On desktop systems, the most common reason for this problem is that Firefox is not set up to work with your security software, specifically the feature that scans your web requests and responses. When you are using a secure site, the software needs to present a "fake" certificate so it can intercept and decrypt those communications. To get a better fix on the problem, could you post the "Technical Details" information on the error page, especially the error code.