I too get system hangs when watching a flash video in firefox. linux-3.8.13,
libdrm, mesa etc. git. Screen simply becomes black (no signal) and machine is
dead, leaving a hard reset as the only option. The dmesg is flooded with the
following lines:

radeon 0000:01:00.0: GPU fault detected: 147 0x0d859002
radeon 0000:01:00.0:   VM_CONTEXT1_PROTECTION_FAULT_ADDR   0x000012D8
radeon 0000:01:00.0:   VM_CONTEXT1_PROTECTION_FAULT_STATUS 0x05090002
[...] repeated a hundred times with only the first line changing a bit [...]

then:
radeon 0000:01:00.0: GPU fault detected: 146 0x07151004
radeon 0000:01:00.0:   VM_CONTEXT1_PROTECTION_FAULT_ADDR   0x00000000
radeon 0000:01:00.0:   VM_CONTEXT1_PROTECTION_FAULT_STATUS 0x00000000
[...] repeated a hundred times with only the first line changing a bit [...]

Times indicate this goes on for approximately two minutes before the hang.