Looks like this is due to bug 742549. NS_IMPL_STRING_ATTR previously called GetAttrHelper, which called GetAttr, ignored the value, and always returned NS_OK. Bug 742549 changed it to return the result of GetAttr, which is a bool, not nsresult.
Er, oops. SetAttr and GetAttr have different return types... The GetAttr case should just always return NS_OK. Want to patch and I'll review?
Created attachment 647115 [details] [diff] [review] Patch
Comment on attachment 647115 [details] [diff] [review] Patch r=me