Soon i will be able to try out a Firewalla gold and i want to use it as router but i do not want to get rid of the RB5009.
I have a fiber connection from my ISP and they at the time installed a Huawei mediaconverter which i immediately removed.
Currently i have my RB5009 connected to the fiber cable directly with a SFP, i want to keep it this way but i do not want to use it as a router when i try out the firewalla box.
I just want to use it as mediaconverter because the rb5009 is powered by a POE switch and the huawei would eat up another powersocket which i do not have.
Can i use the rb5009 just as a simple "Mediaconverter" so i can plug in the firewalla (which does not have a sfp) with rj45 in my mikrotik.

Expensive media convertor ... but yes, quite possible.

Expensive media convertor ... but yes, quite possible.

Temporarily Any idea on how to exactly do this?

Put SFP and ether<x> in one bridge.
Nothing else to be done, I would think.

absolutely viable

have 2 4011 in that configuration and it works quite well.
those 2 4011 are installed temporarily to insert a "tap point" for a POC setup for a palo alto IDS/IPS solution. working transparently and at wirespeed so far
so 5009 surely will handle the load

just create a bridge and add the SFP and RJ45 interface without vlan-filtering
adjust the MTU of the bridge to your needs and you are good to go