Avoid sending browser startup time information to Mozilla
In https://lists.torproject.org/pipermail/tor-talk/2014-May/032889.html, someone suggested disabling the addon metadata queries to Mozilla. The data doesn't seem that harmful (they don't mention tying it to a unique identifier or cookies) but I guess we might as well disable it. I don't like the fact that it sends browser startup time at least. That should be part of Telemetry, not addon metadata retrieval.
The pref is extensions.getAddons.cache.enabled -> false.