Nightly is a future version of Firefox that is getting new code updates on a daily basis. Not stable for normal use.

Metro refers to the tile interface of the Windows 8 home screen, and related user interface changes to applications. I'm not sure on the status of Metro integration support in Firefox.

Nightly is the name of one of four main development channels of Firefox.

The four are Release, Beta, Aurora and Nightly. Release has 26.0, Beta has 27.0b (beta 6 currently), Aurora has 28.0a2 and Nightly has 29.0a1.

Nightly and even Aurora gets checkins pretty much everyday so they get updates each of those days.

For more information about "Metro-style" Firefox for Windows 8, see How do I launch Firefox for Windows 8 Touch?

This is a new feature that is currently only in pre-release test versions of Firefox. If you are using the latest stable release, it does not have this feature included.