Bug 948395 is showing the symptoms of a b2g crash but nobody on the dev team has taken ownership to track it down. It's causing a lot of test failures so over Christmas period when the tree is quiet will be a good time to investigate it. You'll have to run the tests like TBPL does repeatedly until you get the crash. I would dig out a build from TBPL (clicking the 'B' icon) that has exhibited the crash and run it locallly following the instructions I made on MDN. I expect when it crashes you'll probably see the Crash Reporter, you'll need to submit/dig out the crash report and the file a bug as per usual.
These have disappeared and A-team are doing lots of work to make the failures more transparent.