Disable randomised Firefox HTTP cache decay user test groups

I need to look into it more, but I think we should be setting browser.cache.frecency_experiment to -1 to 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.