Assuming you have made all the proxy setting in BurpSuite right, the reason could be that you are not using a proxy on Firefox. To capture packets, firefox should use manual proxy. To set this, open firefox, click on Menu-> Preferences. Scroll down to Network Proxy section and click on "Settings". Choose "Manual Proxy Configuration" and set the proxy as 127.0.0.1 and port as 8080. Save and exit. Now BurpSuite should be able to capture packets.