I had followed the instructions posted by emk2203 (My AMD Radeon graphic card is not working on 16.04) and now I could make the apps use my secondary AMD card and it's work! Look:
Now, runnig Firefox by Terminal with DRI_PRIME=1:
Now I need to know how configure the system to always runs the app with the DRI_PRIME=1 set.
Sorry for bad english. I'm brasilian.