The helper function nsContentUtils::URIIsLocalFile() was added for bug 402983 but the name may lead people into doing the wrong thing. For the purposes of file: uri security checks (bug 402983) we did not want to de-nest jar: uris, but just about anywhere else in the product we will always want to use the innermost URI for these kinds of checks. Options: - rename it to OuterURIIsLocalFile() (and optionally create a de-nesting version - change the signature to include a "de-nest" boolean parameter (and default true) Incidentally we should document why we didn't just QI to nsIFileURL. resource: URIs are also nsIFileURLs and we explicitly want to exclude them in this case. We checked the protocol handler flags instead, but a schemeIs check might have been simpler. Maybe we should have checked for the literal "file:" scheme instead.