|nsJSContext::InitContext| sets up the error reporter for the context to be |NS_ScriptErrorReporter| (both in nsJSEnvironment.cpp) |nsXBLDocGlobalObject|, in nsXBLDocumentInfo.cpp overrides this with its own error reporter, |XBL_ProtoErrorReporter|. The XBL version is significantly simpler. Its not clear to me why it does this, and apparently isn't to Boris either, as he asked me to open a bug on it...

Ah, I see what's going on. NS_ScriptErrorReporter relies on having a window attached to the context to work right... and there isn't one in XBL. Sorry for the false alarm. :(