diff --git a/netwerk/test/unit/xpcshell.ini b/netwerk/test/unit/xpcshell.ini
index 18b44e87ac46..93939c0924d6 100644
--- a/netwerk/test/unit/xpcshell.ini
+++ b/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]