Set network.proxy.failover_direct to false in FF 91

https://bugzilla.mozilla.org/show_bug.cgi?id=1720221 https://phabricator.services.mozilla.com/D119695

To combat malicious add-ons, Firefox has set up a mechanism to have system requests fail over outside the proxy. This is controlled by a pref; and will be uplifted to 91.