The feature landed in today's nightly build of Firefox, and if it sticks it may become part of Firefox 20.
I just landed some interesting code for bug 792438. It should be in the December 6th nightly, and If it sticks it will be part of Firefox 20.
The new feature keeps the network clear of media transfers while a page's basic html ,css, and js are loaded. This results in a faster first paint as the elements that block initial rendering are loaded without competition. Total page load is potentially slightly regressed, but the responsiveness more than makes up for it. Similar algorithms report first paint improvements of 30% with pageload regressions around 5%. There are some cases where the effect is a lot more obvious than 30% - try pinterest.com, 163.com, www.skyrock.com, or www.tmz.com - all of these sites have large amounts of highly parallel media on the page that, in previous versions, competed with the required css.
Any claim of benefit is going to depend on bandwidth, latency, and the contents of your cache (e.g. if you've already got the js and css cached, this is moot.. or if you have a localhost connection like talos tp5 uses it is also moot because bandwidth and latency are essentially ideal there).