@Nandu Bro i want to know, where you import the cerificate, in firefox after setting up manual proxy in network settings go to http://burp and download ca cerficate, To install cerficate go to privacy&security tab in firefox option, and select view certificate option, while in authorities tab you may find old certificate, delete that one and import the newy downloaded and check mark all the conditions, if you do this, you can easily intercept pages, if it is not worked just close and restart the burpsuite again.

Did you remove old certificate, mine is working.

@Nandu yes i got this, in that way also you can import the certificate.

@Nandu Bro i want to know, where you import the cerificate, in firefox after setting up manual proxy in network settings go to http://burp and download ca cerficate, To install cerficate go to privacy&security tab in firefox option, and select view certificate option, while in authorities tab you may find old certificate, delete that one and import the newy downloaded and check mark all the conditions, if you do this, you can easily intercept pages, if it is not worked just close and restart the burpsuite again.