In Jeff Walden's blog post (http://whereswalden.com/2010/04/16/more-spidermonkey-changes-ancient-esoteric-very-rarely-used-syntax-for-creating-getters-and-setters-is-being-removed/) is written the intention to remove __defineGetter__ and __defineSetter__ support in the longer run. Unlike in bug 642500 (remove mutable __proto__), all use cases for __defineGetter__ and __defineSetter__ are covered by a standradized alternative; namely Object.defineProperty.

I think this should be wontfixed. The only thing that happened since Waldo's blog post is that IE was, despite their claims cited in the blog post, forced to implemented them in IE11. We don't have an infinite budget for breaking changes, so out of all the things that would actually simplify the engine, I don't think that the benefit of removing 70 lines of self-hosted JS are worth the cost.

jorendorff, as module owner, can you please make the call?

This was already marked as deprecated. We recently added the "JavaScript aka Web ECMAScript" spec here. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/__defineGetter__ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/__defineSetter__