I rebuilt my kernel, taking AMDGPU out and using "ATI Radeon" instead. Now X works for me, and even my power-off issue is gone.
I don't think it has anything to do with firefox, really, or performance, but rather a problem with the driver and hardware coordination. According to the gentoo AMDGPU webpage, the STONEY processor *should* be supported: 00:01.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Stoney [Radeon R2/R3/R4/R5 Graphics] (rev da)
