Harmony proposal: http://wiki.ecmascript.org/doku.php?id=harmony:observe The API has reached some level of maturity. It's been prototyped on V8 https://mail.mozilla.org/pipermail/es-discuss/2012-August/024547.html
Firebug would need this feature at least at two places: For stopping the debugger as soon as a DOM property was added, changed or removed as well as for reducing CPU load within its Layout side panel by observing changes to the layout properties instead of listening to paint events. Sebastian  http://code.google.com/p/fbug/issues/detail?id=4162  http://code.google.com/p/fbug/issues/detail?id=6336
Landed in Chrome 25: http://updates.html5rocks.com/2012/11/Respond-to-change-with-Object-observe
4 years ago
Would be nice to see come work on this item. Chrome will probably enable this feature by default and Firefox have nothing already. This would encourage devs to build for FirefoxOS since complex architectures should be easily accomplished. I beg Mozilla to put attention on this feature. Thanks
FYI, Object.observe is now enabled by default in V8 and will ship in M35 of Chrome.
guys, really, this is not the right place to advocate for this feature. Bugzilla is only meant to be used for technical discussion, so unless you have them (or patches, we love patches), please, use newsgroups. Read Bugzilla Etiquette point 1.1 - https://bugzilla.mozilla.org/page.cgi?id=etiquette.html
What is the complexity of this item? What is the impediments? How could I start contributing to it?
Diego: if you haven't compiled Firefox before, you should visit the #introduction IRC channel on irc.mozilla.org. Then SpiderMonkey developers on the #jsapi IRC channel can answer questions you have about Object.observe. Note that Object.observe is likely a challenging feature to implement and not a good first bug.
It's 2015 and still nobody is working on this ? How can we help? Can we donate for development of this feature? It's really important when it comes to performance.
https://mail.mozilla.org/pipermail/es-discuss/2015-November/044684.html suggests this is dead.
Thank you for the update. I had not caught wind of that trend, and appreciate you passing it on. I have used polyfills before (even written my own a couple of years ago), but was merely hoping it would be included in the engine for performance reasons. I can understand the logic for removing it, however; and again, I appreciate you passing this information on.
No point in keep this open anymore. Chrome is aiming to remove this API in version 50.
Marked as obsolete on MDN.