BL-676: JScript error in nsBlocklistService.js breaks BigBook

I don't like seeing "should be safe" :-).

In one way, warnings are by definition safe to ignore: they report problems that are not necessarily disasters. I don't know much about JS warnings, so it may be that most of them are really not useful. But it makes me nervous to just ignore information that someone must have thought would help catch problems in our code. Is it a lot of work to deal with the cause of the warnings? Could we be more specific, only ignoring warnings that we know are commonly spurious for us?