Right now we crash in NS_NewTelephony() if the page accessing navigator.mozTelephony is a chrome page because document->NodePrincipal()->GetURI() returns a null URI (the principal is the system principal).
I wouldn't say there's anything critical about this since (a) navigator.mozTelephony is not part of any normal Firefox build and (b) B2G's Gaia UI doesn't use chrome at all.
Criticality and priority are two different things.
Created attachment 587316 [details] [diff] [review] Patch, v1 This has r=sicking.
Created attachment 587842 [details] [diff] [review] Patch, v2 We decided to allow chrome access always.