Seems like there's really no reason to throw an exception in the case where there is no previous page here (i.e. the current page is the first page in a window), returning an empty (void) string would be plenty good IMO. Now, history.previous requires UniversalBrowserRead, so nobody really cares, but still. This would be a good first bug btw, if some wants to chime in. This came up as part of crowder's investigation of bug 426131.
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