I've been running the last three in Firefox on a Tonga system that was
simultaneously used for other tests for 45 minutes now, without a hang.

@Christian: It's a long shot, but by the rough shape of GPU lockup reports over
the last few months I have the impression that the radeon module still has a
lockup bug under pressure (especially with multiple apps running
simultaneously, but that might just be X/the compositor) which was fixed in
amdgpu. Any idea what that might have been?