I'm working to enable E10s on Thunderbird. For a Thunderbird build, XPCShell tests run with a profile that loads the default prefs (which is not the case for Firefox builds) so this test runs with browser.tabs.remote.autostart set to true. It doesn't work. Here's a log:

widget/headless/tests/test_headless.js | Starting test_snapshot
(xpcshell/head.js) | test test_snapshot pending (2)
(xpcshell/head.js) | test run_next_test 0 finished (2)
"CONSOLE_MESSAGE: (error) [JavaScript Error: "NetworkError when attempting to fetch resource." {file: "/builds/worker/workspace/build/tests/xpcshell/head.js" line: 239}]
_do_main@/builds/worker/workspace/build/tests/xpcshell/head.js:239:6
_execute_test@/builds/worker/workspace/build/tests/xpcshell/head.js:567:5

I'll probably end up just turning the test off for Thunderbird. Unless somebody out there has a brilliant idea?

The bug summary is likely wrong. I'd assume the test doesn't work for Firefox either, it's just not noticeable due to bug 1168178 (which would be nice to squash): the tests do not run with what Firefox uses for real.

This test will stop working once Thunderbird enables E10s.

https://hg.mozilla.org/mozilla-central/rev/eae8353a8639