JS 1.7 support was introduced in Firefox 2.0 and is now also available on the server-side via Aptana's Jaxer Server (which uses Gecko 1.9). It introduces a couple of powerful enhancements to the language that would be very welcome in Safari.
These features are also a step towards the JS2 spec. With Gecko, (Mobile)Safari and hopefully later Opera on board, it will become a more viable technique to use on at least domain- and/or mobile-specific sites.
Confirmed as an enhancement request
Our goal is ECMA-262 conformance. Many of the features of JS1.7 are being standardized as a part of ES6 Harmony, those that are not standardized are unlikely to be supported. Duping to the ES6 bug, which is tracking the features in JS1.7 that will be implemented.
*** This bug has been marked as a duplicate of bug 80559 ***