Mozilla today announced the release of the first Nightly build of Firefox Metro. The company also confirmed that the browser is now stable enough for “regular testing” by interested Windows 8 users.

Mozilla’s Director of Community Development Asa Dotzler revealed the news last night:

On Tuesday, our preliminary Metro Firefox development work arrived at mozilla-central, the source code repository that feeds the Firefox Nightly channel. This means that if you are on the Firefox Nightly channel and you have a Windows 8 device, your Wednesday Firefox update should deliver a Metro Firefox tile to the far right end of your Windows Start screen. There’s plenty of work still to do, but it’s stable enough that we’re ready for more and more regular testing.

Unfortunately, as the comments on his post indicate, some users (including yours truly) are having issues enabling the new user interface. A new Nightly build will be coming out tonight though and might fix some of these issues. The new Nightly is supposed to launch a chromeless browser window similar to Metro IE. Here’s how it should look:

Mozilla Firefox Metro Tabs 730x458 Mozilla announces Nightly build of Metro Firefox for Windows 8, says its stable enough for regular testing

You can only access the new user interface if you set Firefoxt Nightly as your default browser. If not, the Desktop version of Firefox will launch instead when you click on the Nightly icon in your Start Screen. This is a limitation Microsoft has purposefully included for security reasons: only the default browser can run in the Metro UI.

The Firefox Nightly channel, as its name implies, is automatically updated each night. The builds are meant only for developers interested in helping out the company test new and experimental features.

After six weeks or so, Nightly builds are brought into the Aurora channel. This is the alpha channel, which is followed by the beta and release channels. We can thus expect the Metro version of Firefox to be released as a stable build somewhere in May or June, but possibly later if testing takes more time.

Mozilla first revealed it had a Windows 8 Metro prototype of Firefox back in April 2012. The company then showed off a pre-release of Firefox for Windows 8 in October 2012.

Image credit: Andreas Krappweis