But some users quickly complained about slow performance and the browser's lack of Flash support.
Mozilla first pitched Firefox 4 to users last August, when it issued an alpha build of what it called 'Fennec'. Later, Mozilla renamed the program to keep it in sync with its better-known desktop browser.
Mozilla touted Firefox 4's new streamlined interface, faster performance and its integrated Sync feature, which lets users running the browser keep bookmarks, passwords and open tabs synchronised across multiple desktop computers and mobile devices.
Some people begged to differ.
"They say it's faster than the stock browser... but it takes twice as long to open," said a user identified as 'Andy' in a review on the Android Market, Google's official app store.
"Has to be the slowest browser in the history of Android," reported 'Oliver' in another review.
Firefox 4 doesn't currently support Flash, but instead relies on HTML5, CSS and other web standards to render video and animated content.
Users also bashed the browser for the Flash omission. "No Flash support. Uninstalled," said 'Rabbi Eli'.
The browser offers a multi-tab interface, uses the same 'Awesome bar' found on the desktop version that automatically retrieves URLs viewed earlier when a partial address or phrase is typed in, and can be customised with a raft of add-ons, another long-time Firefox feature.
Firefox 4 doesn't work on iOS devices such as the iPhone or iPad, as Apple has made it clear it will not approve the browser for the App Store. The best that Mozilla offers for iOS is an app dubbed 'Firefox Home' that synchronises bookmarks, browsing history and open tabs in the iPhone's browser with other copies of Firefox 4.
Unlike last week, when Mozilla touted Firefox 4 downloads for Windows, Mac and Linux, the company has not posted a download scoreboard to track the number of copies Android users grabbed.
Firefox 4 is a free 14MB download from the Android Market, and requires Android 2.0 or later.
Not all Android-powered smartphones will run the new browser, however. For example, several owners of the Motorola Droid reported that Firefox 4 would not work on their devices.