Bill Gates thought he had won the war but it turned out to be only a battle. After seven or eight years of almost absolute dominance of the browser market following the "browser wars", it must have been an unpleasant surprise for Microsoft to find its Internet Explorer (IE) product suddenly under attack again.

The challenger taking a sizeable bite out of Microsoft's 95 per cent share, Mozilla Firefox, is an open-source, free-to-use browser produced by the non-profit organisation the Mozilla Foundation.

Set up in November, it received 10 million downloads in its first month, and after only six months its market share is more than all IE's other competitors combined.

Mozilla's target is to grab 10 per cent of the market by the end of this year - a goal it believes is attainable. If it does achieve that figure, Microsoft will have to sit up and take notice.

IE came from nowhere in the mid 1990s when Microsoft, recognising the commercial potential of Web browsers, took on Netscape Navigator, then the dominant figure with the same sort of market share that IE enjoys at the moment.

Microsoft's plan was to introduce its own browser and set the default page to its homepage, guessing that many people would not know that this could be changed or how to change it. Thus, users logging on would go straight to Microsoft's online shop every time.

So began the browser wars, with the two rivals issuing newer models every six months. Given the fact that IE was bundled in with the Windows operating system, and the vast sources of revenue that Microsoft had compared to Netscape, it was hardly a surprise who won in the end.

Next month, publishers O'Reilly books will issue Don't click on the blue E!, by Scott Granneman. The title is a reference to the instantly recognisable IE logo that appears on the desktop of all PCs running Windows.

For many people, IE is the internet, such is its ubiquity. Mozilla's aim is to let people know that there are alternatives, and Mr Granneman's aim is to help people make the conversion.

Changing to Firefox is a simple process (www.getfirefox.com), and buying a book just to find out how to go to a url and click on a page seems a little excessive. Don't click on the blue E! is primarily aimed at schools, businesses and non-technical users.

But why should anyone change from IE? The two primary reasons are security and useability. As an integrated part of the Windows package, IE is more vulnerable to bugs and viruses, whereas a separate product, such as Firefox, is more resistant, claim its promoters.

Also, given its near-universal usage, IE is a sitting duck for mass attacks from hackers. Anyone wishing to wreak havoc with a new virus has only one product to deal with and widespread propagation is virtually guaranteed.

The cost of one book suddenly seems very reasonable compared to replacing a hard drive riddled with spyware.

The other factor is Firefox's consumer-friendly aspects. Microsoft has not issued a new version of IE in more than three years, which has allowed Mozilla to offer a better product.

Firefox's interface is clean and simple and it features added extras not available with IE, such as tabbed browsing and an integrated and effective pop-up blocker.

Tabbed browsing allows users to keep several pages open within one browser window and quickly switch between them. This enables a fast comparison of search results or different online shops, and also allows users to open all their favourite bookmarks with one click.

It is not just the fancy features or the slick appearance that is driving Firefox's popularity. The market upstart has benefitted from an aggressive market campaign, word of mouth and, to a lesser extent, anti-Microsoft sentiment.

Taking its cue from the Howard Dean online campaign in the US presidential election, Mozilla has set up www.spreadfirefox.com, where interested parties can find out more, chat in forums and donate to the cause.

Crucially, Mozilla is also supported by Microsoft competitors such as America Online (AOL), Sun Microsystems and IBM. AOL alone gave $2 million (€1.53 million) to the project.

It is this marketing that has lifted the new kid on the block above the forgotten son, Opera. For years Opera was the nearest thing IE had to a competitor, and it was the first to introduce tabbed browsing, the integrated pop-up blocker and the password wand.

Now it has had to watch while Firefox steals its ideas and its thunder.

Opera chief executive Mr Jon von Tetzchner is philosophical about the turn of events. "Obviously we would have liked it if Opera got more media attention," he says. "At the same time, we believe it is good news that there is more interest in browsers. We now need to work hard to make the most of this opportunity. We believe we have a product that can handle comparison very well."

Opera is extending its operations, with new offices in Asia and North America this year, and will continue its record for innovation with voice interaction, where users can control their browser by talking to it.

Such is the momentum of Firefox, it is tempting to say that after all those years of waiting for its chance, Opera has missed it.

Although it cannot be written off, the introduction of voice interaction reflects its focus on hand-held devices and mobile computers and away from the desktop market.

Firefox's timing couldn't be worse for Microsoft. Having become so comfortable with its dominant position, Microsoft discontinued its IE for Macs and suspended new versions of IE until the next version of Windows, code named Longhorn, is released, probably in early 2007.

The long delays in releasing Longhorn has left it open to attack.

Plus, with the release of the Mini Mac and Apple's high profile at the moment, iMac and Power Mac users are increasing in number, and the last version of IE is a clunky nightmare on the latest Apple operating system.

Mr Eric Clarke is IT co-ordinator at the Royal College of Surgeons. The college runs a compulsory Apple laptop purchase programme and recommends its students to use Firefox.

"We had been happy using IE until... about two years ago, Microsoft decided that they would stop making Explorer for Mac," he says. "So some sites, for example internet banking, recommended using a browser with higher specs. We found that Firefox was the most suitable product, especially with our e-mailing product that we had problems with under IE.

"A while ago there was no real reason to change because all browsers were the same, but Firefox seems to be offering something different."

Mac users still only account for a small percentage of the market, however, and Firefox's growth will have to slow at some stage. It may even plateau once all the techies and anti-Microsoft brigade have been tapped. For many people, the "blue e" will remain the doorway to the Web, despite the security concerns.

Microsoft's official line is that it welcomes all competition, especially on the Windows platform. Whether or not Mr Gates is losing sleep, the increased competition is fantastic news for consumers.

Innovation had stagnated since the end of the browser wars and another contest can only mean improved products on the market. Unlike Netscape, Mozilla is not out for commercial gain but to provide the best Web experience.

There may be more joining the battle before too long. AOL plans to release an updated version of Netscape Navigator, which it bought in 1998 for $4.2 billion.

Not to be outdone, another Silicon Valley giant last week hired Mr Ben Goodger, the chief developer of Firefox. Goodger at Google? The long-rumoured G-Browser may yet become reality.