The MDR weapon talent "Distracted" is
Multiplicative, and so is the Firecrest set bonus for damage to targets
on fire.
Source
The Division | The Guide To Higher/More Damage | Additive vs Multiplicative
https://youtu.be/iPZ_LdUZ17Q
The Division | Guide to More/Higher Damage | Additive vs Multiplicative pt2
https://youtu.be/ukt_wCxWbc0
It's also important to understand what firearms scaling is and how that effects your damage. Oddly enough the MDR has a really high firearms scaling. Translation; low firearms builds are a poor way to run the MDR. Funny enough low Firearms (Electronics stacked) is not the best way to run Firecrest even though the 6 PC makes it look that way. At least with the MDR. I run 7.5k Firearms 4k Stam 4.7k Elec for my Firecrest build. Competent / Determined on the MDR and Dragons breath turret, and Flashbang sticky for skills. It's a great way to play FC. Really wish the 6pc FC bonus gave flash sticky a fire effect instead of the explosion. It would be awesome.
See this for info. Google Division firearms scaling for the spreadsheet if you want to calculate the scaling for individual weapons.
The Division | Is %Damage on Gloves A Buff or Nerf? | Patch 1.8 PTS
https://youtu.be/2IZLOC0Icbo