Often public hotspots require you to authenticate before you can browse. When they redirect from your intended address, Firefox often objects to a fake SSL certificate. Is that what you're experiencing? The best workarounds are: (1) Let Android handle the authentication page outside of your browser. This usually happens automatically on my device but it may vary between versions or manufacturers. (2) Go to an insecure address like http://example.com to satisfy the hotspot before proceeding to sites you really want to visit.