Ensure Burp is started and running and then execute the following steps:

1. Open the Firefox browser to the  http://burp URL. You must type the URL exactly as shown to reach this page. You should see the following screen in your browser. Note the link on the right-hand side labeled CA Certificate. Click the link to download the PortSwigger CA certificate:

2. You will be presented with a dialog box prompting you to download the PortSwigger CA certificate. The file is labeled cacert.der. Download the file to a location on your hard drive.
3. In Firefox, open the Firefox menu. Click on Options.

4. Click Privacy & Security on the left-hand side, scroll down to Certificates section. Click the View Certificates… button:

5. Select the Authorities tab. Click Import, select the Burp CA certificate file that you previously saved, and click Open:

6. In the dialog box that pops up, check the Trust this CA to identify websites box, and click OK. Click OK on the Certificate Manager dialog as well:

Close all dialog boxes and restart Firefox. If installation was successful, you should now be able to visit any HTTPS URL in your browser while proxying the traffic through Burp without any security warnings.