consider enabling new cache isolation features

From #33534 (closed): Firefox 78 offers several new first-party isolation features (off by default). Should we enable them?

• https://bugzilla.mozilla.org/show_bug.cgi?id=1639807
  • Isolate image cache per first-party when privacy.partition.network_state is set to true)
• https://bugzilla.mozilla.org/show_bug.cgi?id=1639154
  • Isolate network cache per first-party when privacy.partition.network_state is set to true)
• https://bugzilla.mozilla.org/show_bug.cgi?id=1639247
  • Isolate DNS cache per first-party when privacy.partition.network_state is set to true)