[tor-bugs] #13575 [Tor Browser]: Disable randomised Firefox HTTP cache decay user test groups

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#13575: Disable randomised Firefox HTTP cache decay user test groups
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 Reporter:  isis                          |          Owner:  tbb-team
     Type:  defect                        |         Status:  new
 Priority:  normal                        |      Milestone:
Component:  Tor Browser                   |        Version:
 Keywords:  tbb-pref, tbb-fingerprinting  |  Actual Points:
Parent ID:                                |         Points:
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 I need to look into it more, but I think we should be setting
 `browser.cache.frecency_experiment` to `-1` to
 [https://bugzilla.mozilla.org/show_bug.cgi?id=986728#c3 disable
 HTTP_CACHE_MISS_HALFLIFE_EXPERIMENT]. Since we have Telemetry disabled, of
 course, Mozilla's experiment shouldn't be collecting any data on our
 users. However, if that pref is not set in `firefox.js` to `-1`, it will
 default to `0`. And if it's `0`, then it's randomised between `1` and `4`
 inclusive, setting different HTTP cache decay times for the four groups,
 which might make for a bit of a rough-edged fingerprinting mechanism.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/13575>
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