A version of Firefox has been available for Android since March, and since then a few updates have been released that have made them more stable. I have Firefox installed on my HTC Flyer and I am using it as the primary browser on that device because I like how it renders pages on the tablet.
If you search the Android Market for Firefox, you will see two versions, the release version and the Firefox Beta. You can have both installed on your device at the same time. While the Firefox Beta in the Market is pretty much the same as the release version, Mozilla is actively working on an entirely new version of Firefox for Android that will be available in their nightly build for developers tonight for the first time.
The versions of Firefox available for Android up to this point have used the same technology as the desktop version of the app, which means it supports extensions like the desktop app, but it doesn’t perform as fast as it could on Android. The overhauled version of Firefox uses some of Android’s built-in technology to provide faster performance.
While this version will use less memory and will be more responsive, it will not support the current add-on capabilities of Firefox, which means developers will have to re-write their extensions. The nightly release of this Android-native version of Firefox also does not support Sync, which is how you can share bookmarks, passwords, and tabs between the desktop version of mobile versions. Sync will be added to this version in December.
If you want to try the nightly builds the process to get them is pretty straight forward. You need to enable your phone to install applications from unknowns sources, then download the app using the web browser. After it installs you will receive notifications of future updates. Before you install the nightly build you should be aware that this build represents what Mozilla is actively working on, will change frequently, and will have bugs and missing features.