Burpsuite: just passthrough firefox detect portal
Disable Captive portal from firefox as seen in this link.
In order to do that open a new tab and type
about:config, then search for
network.captive-portal-service.enabled and set it as false then you are good to go.
When you get a request in BurpSuite that you don't want to intercept again, click the "Action" button, followed by "Do not intercept...", and choose "requests to this host". This should solve the problem without modifying Firefox.