ftp pages are not isolated in the cache
I noticed that if I load http://arthuredelstein.github.io/tordemos/ftp-in-iframe.html and http://arthurtordemos.github.io/tordemos/ftp-in-iframe.html in two different tabs.
Then I can see that the ftp document, ftp://ftp.mozilla.org/pub/mozilla.org/ in about:cache. It says "Fetch count: 2" and "No expiration time."
The behavior is inconsistent. Sometimes it looks as though there is only one network fetch, and sometimes two. Also, the ftp page sometimes disappears from the cache after a short while, which makes me think "no expiration time" means "expires pretty soon". The quick expiration means this is likely not a great linking vector, but I guess it's probably better to key ftp page caching by url bar domain.
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