Mozilla has been keeping to a tight schedule of having a completed Firefox release every five to six weeks, and it's very much on track. The browser team's Ehsan Akhgari has confirmed that a properly polished version of Firefox 15 should reach the download servers on August 29th. When it does arrive, the new release will primarily expand the silent updates that Windows users first saw in Firefox 12: future iterations on all platforms will install themselves in the background and should be truly ready to go the next time the browser starts. Beyond this deliberately subtle change, the finished version 15 upgrade should still support Opus audio as well as clamp down on out-of-control memory use from add-ons. We're looking forward to not noticing the differences very shortly.
Update: Although it's not on the front page yet, both Mozilla, reader Josh and this writer can confirm that Firefox 15 is rolling out sooner than expected -- there's no reason to wait.
Update 2: It's now easier to get a stand-alone copy if you're not updating, since Mozilla just updated the Firefox front page to reflect the new version. Android users are also getting an update through Google Play that brings earlier speed updates to tablets, a personalized start page and a whole host of extra fixes, some of which come directly from the desktop Firefox 15.