To celebrate, the Mozilla Foundation, which led its development, is giving away commemorative coins to people who have done the best job promoting it.

The 50 coins were specially designed by one of the Foundation's volunteers.

Firefox is Microsoft Internet Explorer's (IE) main rival. Many like it because it is easily customised, while others like its tabbed browsing.

This means that instead of opening a new browser window for each page a user wants, it opens a new tab in the same single page.

Although Firefox has been downloaded 50 million times, this does not necessarily mean it has the same number of individual users.

But Firefox has clearly gained ground on IE. It has so far not had as many security problems as IE, and when there has been a problem, it has taken less time to fix because it is an open source piece of software.

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This means that anyone with the right skills can access and modify the code.

It also means people are free to adapt the software's core code to create other innovative features, such as add-ons, RSS news feed readers, or extensions to the program.

Spreading it

The coins are to say a "thank-you" to those who have actively promoted the open source browser to a wider audience, through the SpreadFirefox campaign website, said Mozilla.

On the site, users are encouraged to promote the browser through their own websites and word-of-mouth.

There have been other preview versions of Firefox, but version 1.0 was the first complete official program and was released on 9 November.

Firefox said earlier this year that it wanted 10% of the browser market by the end of 2005.

However, other browsers, such as Opera and Apple's Safari, are also challenging Microsoft's domination.

Microsoft is releasing its next-generation IE7 later this year, too, which it promises will be more secure.