I got the entire setup to work except for the following problem.
Firefox is only able to use the proxy when there is at least one other network adapter [e.g. my lan] is connected. In case the LAN is connected Firefox does not use the LAN but uses the proxy. So all my proxy settings are correct, adb is running well and the phone is actually proxing the data.
However the moment I diconnect the LAN, firefox comes up with a message stating "Offine Mode" ... see image below...
Also below are the proxy settings from the about