Firefox has been decoupled from the system certificate store since the
nss p11-kit integration in combination with our cacert package does not
expose CKA_NSS_MOZILLA_CA_POLICY, which among other things is required
for addon updates.
This test was failing because Firefox was displaying a download prompt
rather than the page content, presumably because mumble mumble
content-type sniffing.
By explicitly setting a content-type, the test now passes.
