Available on the Developer Portal now, the beta releases ask developers to focus testing on a large number of areas, including website and extension compatability, adding Google accounts to Internet Accounts, and navigation using VoiceOver.
As far as version numbers are concerned, the betas clock in at 7.1.5 and 6.2.5 for OS X Mavericks and OS X Mountain Lion respectively. If you're an interested developer, you can read the full list of focus areas and snag both releases from the Developer Portal now.
Source: 9to5Mac
