Does anyone know how much the starship sensors skill affects Fire on my mark ability?

Hop onto the Tribble server and test it

starship sensors only affects the anti stealth part of fire on my mark...the dmg debuff is uneffected. There is nothing that can increase the dmg debuff.
personally i think that only for the resistance to Confuse and Placate effects it is a too expensive skill to invest into. PVP players may want to put 3 points into it, but even there it is questionable. Stealth detection is useless anyway, also only valid for PVP and not even sure about that.

also you will find this usefull for other skills.

Detection works in PvE and PvP. It's not great, but it works.

My fleet mate used to be able to detect my Defiant from 5km+ when he tried.

I've been able to find players who attempted to cloak to run away. I've also found NPCs that refuse to decloak and prevented me from continuing with the mission.

But, there are far better things to invest in.