webui: use manual Firefox configuration for Firefox >= 40
The intended course of action is to show manual configuration in
browserconfig.html instead of configuration with the extension
for versions of Firefox >= 40.
The reasoning is:
* plan for enterprise environments was not published yet which
forces as to use AMO (addons.mozilla.org)
* with AMO the user experience is worse than a manual configuration
steps for AMO:
* go to AMO page
* installed the extension
* go back to IPA page
* probably refresh
* click configure
* confirm
manual config:
* go to about:config
* set network.negotiate-auth.trusted-uris with *domain.name
https://fedorahosted.org/freeipa/ticket/4906
Reviewed-By: Martin Basti <mbasti@redhat.com>