Yes I can see how this might seem confusing Let me see if I can clear things up a bit.
0.5.3 was announced late due to some missing tools (Firmware Manager), but has been out on the Web IDE now for about a month now.
0.6.0 is locked down and waiting to be released. We are working on the Firmware Manager for it, and it will also support Libraries v2.0.
0.7.0 is currently in development and the first prerelease of that is coming out after 0.6.0 is set as the new default firmware.
We were in fact working on 0.6.0 and 0.5.3 at the same time. The idea around this is we are currently experimenting with the idea of back porting critical bug fixes to 0.5.x, while continuing new feature development in future releases. We don’t currently have a timeline around when and how often we will add to 0.5.x line of firmware, but so far it’s helped to make 0.5.3 one of the most stable firmware releases currently out.
Critical fixes or known issues to 0.5.3 would definitely be slated to be fixed now in 0.7.x since 0.6.0 is locked down pending release. Those fixes if desirable and applicable to 0.5.x users could then be back ported to 0.5.x. If you have reproducible firmware issues, please report them in our firmware issue tracking system on Github.
There should be no backwards incompatible API compatibility issues between these versions, i.e. future versions should continue to support existing APIs. There may be breaking changes from time to time, and APIs can be depreciated over time in favor of new ones. Those changes will be documented in the Changelog and Release notes for each firmware release.
Hopefully this clears things up. Please let me know if you need more info!