If you have previously installed a different CA certificate generated by Burp, you should first remove it by following the tutorial for removing Firefox’s CA cert.
With Burp running, visit http://burp in your browser and click the "CA Certificate" link to download and save your Burp CA certificate. Take note of where you save the Burp CA certificate.
In Firefox open the Firefox Menu.
Click on "Preferences / "Options"".
Go to the "Privacy and Security" settings.
Click "View Certificates".
Select the "Authorities" tab
Click "Import", select the Burp CA certificate file that you previously saved and click “Open”.
In the dialog box that pops up, check the box "Trust this CA to identify web sites", and click "OK".
Close all dialogs and restart Firefox.
If everything has worked, you should now be able to visit any HTTPS URL via Burp without any security warnings.
Ensure that the certificate is trusted to identify web sites.