E4X was also supported by SpiderMonkey (used in Firefox and Thunderbird), but has been removed. In Firefox 10, E4X syntax was no longer accepted in SpiderMonkey when ECMAScript 5 "strict mode" is enabled.[3] According to Brendan Eich, "This thus signals start of deprecation for E4X in SpiderMonkey."[4] and "has been disabled by default for webpages (content) in Firefox 17, disabled by default for chrome in Firefox 20, and has been removed in Firefox 21"[5]