10:58 AM

Extensions Are Key To Firefox Success

Mozilla's developers built Firefox from the ground up to give third-party extension developers room to run. The results have been more successful, and more vital to the open-source browser's long-term prospects, than any of them could have imagined.

"I noticed that I'm not good at typing in URLs -- I'll type '.rog' instead of '.org," recalls Ben Karel, a freshman at the University of Delaware. "One day I thought, 'Wouldn't it be cool if Firefox could fix those stupid little mistakes for me?'" So he wrote an "extension" for the Mozilla Foundation's open-source Web browser to do just that.

Today, Mozilla's official extensions Web site lists around 600 add-ons for Firefox alone; other extensions target the foundation's Mozilla software suite, Thunderbird email client, and a handful of smaller projects. Extensions vary greatly in size, complexity, and purpose: Some, like Karel's spell-checker, are compact, single-purpose widgets, while others are bigger, more complex mini-applications created and maintained by teams of developers. Together, they offer something for everyone - including the Mozilla Foundation itself.

Room To Run

As popular as the extension model has been among developers, it has turned into a genuine phenomenon among Firefox users. The most popular extensions boast more downloads than most desktop software products: Users have downloaded FlashGot, a customized download manager, more than 12 million times from the Mozilla site alone, and a number of other extensions can claim more than one million downloads. Judging from comments posted on various online forums, at least some Firefox users have adopted the browser in order to use specific extensions - and they will avoid versions of Firefox that fail, for reasons discussed below, to support their favorites.

Mozilla recognizes and appreciates the nature of its relationship with extension developers, said Karel. "Firefox's developers have been very, very supportive of extensions. I think pretty much everyone involved with the Firefox project is well aware that extensions are a major factor in Firefox's adoption." Karel also contributes to Adblock, an extension which effectively strips out banner ads from Web pages and has itself been downloaded from the Mozilla site more than 3.5 million times.

The original Mozilla Application Suite, according to developers, did not provide a robust enough set of APIs for installing, removing, and managing extensions. When the Mozilla developer team started work on what would become Firefox, however, they made removing this limitation one of their first and highest priorities.

That up-front effort paid off nicely, according to Asa Dotzler, Mozilla Foundation community coordinator. "Providing comprehensive extension management and improved developer APIs was a key factor in the explosive growth of the extension community," Dotzler said, "and has allowed us to continue to support hundreds of niche features without cluttering up the Firefox feature set."

The process of writing an extension can range from a relatively trivial scripting exercise to a hardcore programming challenge. The basic prerequisites, however, include solid working knowledge of both Javascript and HTML; these, in turn, are helpful when learning the XML User Interface Language (XUL), which defines the user interface elements for extensions as well as for Firefox itself.

"I had no experience developing for Firefox at all. But with some Javascript knowledge and after reading some tutorials, I had enough information to put it all together," says "Daniel,"a computer consultant who built and maintains the popular CustomizeGoogle extension, which applies client-side scripts to render Google's sites and services more convenient for Firefox users. (The benefits of this arrangement appear to be lost on Google's attorneys, whose interest in Daniel's work prompted his request not to print his last name.)

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