Now that Fx 27 has landed in Aurora channel, more people will have access to this feature. It's still off by default, but can be enabled in about:config by setting
browser.cache.use_new_backend to 1
When it is enabled by default (whenever that may be) user's cache directories (and structure) will change to %LocalAppData%\Mozilla\Firefox\Profiles\*\cache2
Just a heads up for the devs, I think support for this can be implemented before the feature actually ships (which will cater to nightly/aurora users of it)
At this point, I don't know if the new cache backend will go on to supersede the current cache directory (foregoing \cache2\ for \cache\ once it ships), but I'll update if I find out anything on that front.