verify that our external helper apps patch is still effective

From #33534 (closed): Attempts to navigate to an unknown protocol using methods such as location.href or are now blocked. We should verify that our patch for #19273 (closed) and related bugs is still effective and that the new behavior does not allow web pages to more easily detect whether a protocol handler is installed.

https://bugzilla.mozilla.org/show_bug.cgi?id=1528305
"Behavior on meta and location.href redirects to an unknown protocol can break pages."

https://www.fxsitecompat.dev/en-CA/docs/2020/navigation-to-unknown-protocol-will-be-blocked/