Let's say I'm trying to download a web page (complete) from The Wall Street Journal with both stock Mozilla Firefox Quantum current (57.0.4) and the PortableApps.com version. Arrive through Twitter, get through the paywall; here's your free WS article: https://twitter.com/GaryPR7/status/950014058404564993
Trying to download the above article produces the results I've just described. Success with stock Firefox, failure with portable Firefox. No add-ons, everything is set to default.