Investigate "No last modified time" entries visible in about:cache
Today I stumbled over entities in the browser cache that were not isolated to the first party domain and contained a Last Modified date "No last modified time (bug 1000338)". They are all resources loaded when visiting www.torproject.org.
The referenced bug number and https://bugzilla.mozilla.org/show_bug.cgi?id=1119406 might be starting points to understand what is going on (although the latter is supposed to be fixed since ESR38)
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