First off, I will offer full disclosure before I continue with this post. I currently own the Motorola Droid, which has a known bug that causes the system to completely freeze that looks to be something with Motorola and a lack of sufficient memory in this device. That has been my main experience with this so I may be overly critical, but I have tested this on a Motorola Droid X which does not have that bug. Performance wise a completely up to date version of the original Droid is the only version to suffer issues, while Samsung Galaxy S and Nexus S may suffer issues if the phones have not been updated. Additionally, starting the application for the very first time may be a little slow as installs some files, but every start after that will be faster.
All that aside, at the time of this writing Firefox is available for all devices running Android 2.0 or above, and the Nokia N900. As with the case of Opera Mobile, Apple’s Terms of Service prevent Mozilla from releasing a version for iOS, but there is a simplified version for the iOS called Firefox Home that offers access to your desktop bookmarks, history, and open tabs.
Firefox on mobile (sometimes referred to as Fennec in some regards) is very similar to Firefox on the desktop. It runs the same rendering engine that you are used to on the desktop, and even offers the ability to sync your bookmarks with your desktop version. Like the desktop version, the mobile version also offers extensions for your browser. Extensions authors need to check off and verify that their extension is compatible with the mobile version, so some desktop extensions may not be available on a mobile device, but many popular extensions are still available. Browser plug-ins such as Flash are not currently available at the time of this writing, but they are working on it.
Firefox mobile offers focuses on giving you as much room as possible for viewing a page. Tabbed browsing is done in a slightly different manner than other mobile devices in that tabs appear on the side of the browser and require the user to swipe the browser to see all of their open tabs on the left. Swipe the browser the other way and you can quickly add/remove bookmarks. Swipe upwards and you can access a search/address bar (aka the awesome bar). The awesome bar behaves just like it does on the desktop where it will search your previously visited pages and display an auto complete so you can just click on that to access it again and save on having to type everything out, a big win on mobile.
All in all it is a pretty decent offering from Mozilla. It offers everything you would expect out of them and is a pretty handy browser.