### 1: Open preferences, click on "Advanced", then select"Encryption":

<img src="@!urlTo('firefox3.jpg')!@"/>

### 2: Click "View Certificates", "Import", and select the certificate file: 

<img src="@!urlTo('firefox3-import.jpg')!@"/>

### 3: Tick "Trust this CS to identify web sites", and click "Ok":

<img src="@!urlTo('firefox3-trust.jpg')!@"/>

You should now see the mitmproxy certificate listed in the Authorities tab.