Bug 1749403 - disable test_cookies_thirdparty.js for Thunderbird. r=necko-reviewers,valentin DONTBUILD

Thunderbird tests run with fission enabled, and this test doesn't work with it enabled.
Flags to disable fission are ignored.

Differential Revision: https://phabricator.services.mozilla.com/D136876

netwerk/test/unit/xpcshell.ini

@@ -200,6 +200,8 @@ skip-if = os == "android" # Bug 1700483
 [test_cookies_read.js]
 [test_cookies_sync_failure.js]
 [test_cookies_thirdparty.js]
+skip-if = appname == 'thunderbird'
+reason = Thunderbird runs with fission enabled. This test requires fission.autostart=false. Bug 1749403.
 [test_cookies_thirdparty_session.js]
 [test_cookies_upgrade_10.js]
 [test_dns_cancel.js]