I am using internet by connecting mobile phone to my laptop using usb. From yesterday,my browser (firefox) shows for all most all website. Why this happen? How can i solve this?
The date/time on your device maybe invalid.
All SSL certificates have a start and end period. Certificates can be compromised and its a good practice to renew them.
Your mobile provider might be routing everything thru a proxy server that is doing SSL inspection, so it can do content filtering or bandwidth optimisation, which Firefox is detecting as a man-in-the-middle attack.