[Tracking Requested - why for this release]:
Central as Beta: https://treeherder.mozilla.org/#/jobs?repo=try&group_state=expanded&revision=b96cb60c2ae0945fe03debdb10cc99691a540efe&searchStr=xpcshell&selectedJob=274798090

Failure log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=274798090&repo=try&lineNumber=3275

[task 2019-11-06T11:55:52.053Z] 11:55:52 INFO - TEST-START | xpcshell-remote.ini:toolkit/components/extensions/test/xpcshell/test_ext_proxy_config.js
[task 2019-11-06T12:00:52.053Z] 12:00:52 WARNING - TEST-UNEXPECTED-TIMEOUT | xpcshell-remote.ini:toolkit/components/extensions/test/xpcshell/test_ext_proxy_config.js | Test timed out
[task 2019-11-06T12:00:52.053Z] 12:00:52 INFO - TEST-INFO took 300000ms
[task 2019-11-06T12:00:52.054Z] 12:00:52 INFO - >>>>>>>
[task 2019-11-06T12:00:52.054Z] 12:00:52 INFO - (xpcshell/head.js) | test MAIN run_test pending (1)
[task 2019-11-06T12:00:52.054Z] 12:00:52 INFO - (xpcshell/head.js) | test run_next_test 0 pending (2)
[task 2019-11-06T12:00:52.055Z] 12:00:52 INFO - (xpcshell/head.js) | test MAIN run_test finished (2)
[task 2019-11-06T12:00:52.055Z] 12:00:52 INFO - running event loop
[task 2019-11-06T12:00:52.055Z] 12:00:52 INFO - xpcshell-remote.ini:toolkit/components/extensions/test/xpcshell/test_ext_proxy_config.js | Starting check_remote
[task 2019-11-06T12:00:52.055Z] 12:00:52 INFO - (xpcshell/head.js) | test check_remote pending (2)

:baku could you please look into this? Thank you.

Hi Andrea,
Based on a brief look to the failure logs and the changes applied in Bug 1593651 to the test_ext_proxy_config.js xpcshell test,
it looks that the test file is just missing a call to AddonTestUtils.overrideCertDB():

diff --git a/toolkit/components/extensions/test/xpcshell/test_ext_proxy_config.js b/toolkit/components/extensions/test/xpcshell/test_ext_proxy_config.js
--- a/toolkit/components/extensions/test/xpcshell/test_ext_proxy_config.js
+++ b/toolkit/components/extensions/test/xpcshell/test_ext_proxy_config.js
@@ -15,6 +15,7 @@ const {
 } = AddonTestUtils;
 
 AddonTestUtils.init(this);
+AddonTestUtils.overrideCertDB();
 
 createAppInfo("xpcshell@tests.mozilla.org", "XPCShell", "1", "42");

This change should be enough to make sure that when the test case is running in a beta build (where the signature checks are enforced) the test extension is still recognized as privileged (on both nightly and non-nightly builds).

https://hg.mozilla.org/mozilla-central/rev/d08a84b0d0b0