There is a problem with HTTP Auth dialogs not appearing when privileged content processes are turned on. See bug 1557457. As such we should turn off the preference browser.tabs.remote.separatePrivilegedContentProcess for both Marionette and geckodriver.

This workaround can removed again once the underlying bug in Firefox got fixed, and privileged content processes is enabled by default for riding the train (bug 1513045). I will add the dependency to this bug too, so that we don't forget to remove the preference.

Also this should block our final Marionette client and harness releases on bug 1534702.

Due to bug 1557457 it can happen that loading a web page with eg.
a HTTP auth prompt, no modal dialog will be shown by Firefox. As
such disable this preference for now globally, until all the issues
with privileged content processes have been solved.

https://treeherder.mozilla.org/#/jobs?repo=try&revision=9bb444b08a5ef18dd29c4ade12bc8ed582ea4037

https://hg.mozilla.org/mozilla-central/rev/bbc2f71bfa7f

Please uplift this test-only patch to mozilla-beta for fixing issues in Marionette with authentication prompts.

https://hg.mozilla.org/releases/mozilla-beta/rev/a8b7e99e3f6c