The newest nightly builds of Firefox Mobile for Android phones are fast, stable, and — unlike the previously released alpha we told you about last month — actually usable.

Development on Firefox for Android is progressing rapidly, and there are a lot of small tweaks and changes to be found in the new nightly builds. But the big news is that everything actually works now. The browser’s performance is much improved, especially in responsiveness, scrolling and zooming.

You can download it here. But be sure to read the release notes, which cover the system requirements (Android 2.0 and up) and the known issues.

This little browser called Fennec (as the mobile version of Firefox is still known at this point in its life) first arrived on Android phones earlier this year. I took it for a spin when the alpha was released in August, and while I noted it had already come a long way in a short time, I was both perplexed and disappointed after a spending a couple of days with it.

I was left wanting because, having seen just about every iteration of Firefox over the years, and having had a wonderful experience testing the Maemo Linux release of Fennec on a Nokia smartphone, I was used to Mozilla shipping alpha versions that were fast, innovative and left you really pumped about the final product.

Not so with this little guy. The first alpha version of Firefox for Android was slow. Really slow. And buggy. Zooming and scrolling were choppy. The Wired home page would mysteriously reload every 20 seconds, and some sites wouldn’t load at all. I double-checked my Nexus One’s system settings, thinking something must be wrong. Since it was alpha code, I planned to revisit it later and measure the changes.

Then I saw this tweet by Mozilla’s Mike Beltzner Friday morning, and I decided it was time.

This most recent nightly build of Firefox for Android fixes most of the performance issues. still doesn’t fare too well (probably our fault), but surfing the rest of the web is much more pleasant in the new Fennec. Scrolling and the pinch-zoom gesture are about as fast as Android’s stock WebKit browser. Page rendering is a touch slower in Fennec than in the Android browser, but we can expect that to improve.

As with the previous releases, Fennec syncs up with your other versions of Firefox, so your history, Awesomebar searches, auto-fill form data and passwords will be the same as you move from desktop to mobile and back again throughout your day. Another cool feature is the unique side-to-side swipe action, which brings up menus for things like tabs, bookmarks and settings. It minimizes the browser chrome and leaves more screen real estate for web pages.