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"".
Select the "Advanced" tab. Select the "Certificates" tab and 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.