Some time ago (bug 847566), we disabled HTTP cache for DEBUG build. This was understandable back at the time. Nowadays it's not needed anymore because httpd.js sends the header "Expires: -1" which basically tells Firefox to always revalidate (it was Bug 939729). However using the cache we can get nice 304 answers instead of downloading everything again.
Comment on attachment 8513704 [details] [review] github PR Hey Ricky, this is part of an effort to use caching for unit tests, so that it's running faster. Next in my list is bug 973873 :) (I filed the intermittent in bug 1091453, it's likely unrelated to this patch)
Comment on attachment 8513704 [details] [review] github PR LGTM. :) r+