Mozilla has a lot on its plate at this point, with it working on a full-blown operating system, but it's handling everything rather well. For example, progress on the Metro (no longer called Metro by Microsoft) version of Firefox is going well.
Not well enough to see a
Modern UI Firefox at Windows 8 launch or even soon after, a beta is scheduled for Q4 2012, but well enough that adventurous users can play around with the Metro version and find it mostly usable.
Having a totally new UI, built on a new set of APIs is work enough, but Mozilla also had to adapt Firefox to work with a touch interface.
For example, it had to build a way for Firefox to differentiate between events triggered by mouse input and touch input and handle those accordingly. It also had to rebuild features like the AwesomeBar to work with the new UI paradigm, the touch keyboard and so on.
Another change that had to be made was implementing an asynchronous file picker, which it retrofitted to all platforms, to make Firefox work with Windows RT APIs.
Plenty of other work has been done to ensure that the features you know and love work in the Metro UI version of Firefox. That said, there's still plenty of work left.
For example, Flash doesn't work in full-screen, though the fact that it works at all is a step forward. Also needed is a way of syncing changes and preferences between the Metro UI version and the desktop version of Firefox, even if they're running on the same device.
What's more, there are several features which are beyond the scope of the first releases, like add-on support. In fact, all of these changes aren't even available in the Nightly version of Firefox. The plan, for now, is to have a "preview" version ready by Q3, at Windows 8 launch. This may not necessarily be an Aurora version, at least not at first. A beta version of the Metro UI Firefox should be landing by the end of the year.