[By Duncan McLeod] For a number of years now, Mozilla’s Firefox has been a popular and growing alternative to Microsoft’s Internet Explorer. It’s estimated that the open-source browser is used by a quarter of all Web users. But its star could be fading.

The rise in Firefox’s popularity has been astonishing if one considers that it is not included as the default browser on Windows or Apple Mac systems.

Most computer users are reluctant to fiddle with their computers and so many simply use what’s included with the operating systems on their machines — and for Windows users, that means Internet Explorer (IE).

As a result of this, the Microsoft browser became the way most people accessed the Web. This is in spite of the fact that for many years IE was an inferior product.

It’s amazing, but a significant number of people still use Microsoft’s ancient IE6 browser, released in 2001. IE6 is bad software. It’s inherently insecure and does not conform to basic Web standards. Yet some very large companies continue to use it.

Given corporate lassitude and consumer indifference then, it’s amazing that Firefox has chalked up 25% of the worldwide market. It’s taken time, but users have slowly switched from IE to Firefox, lured by features not offered in IE, especially in older versions of the Microsoft browser, as well as the ability to extend Firefox’s functionality using third-party plug-ins.

Given that Firefox has proved so successful in wresting a large chunk of the market from IE, it would be fair to assume it will continue to gain market share. But I don’t think that’s going to happen. I think Firefox is going to lose market share in 2010 and it could even slide into irrelevance within a few years.

Microsoft is working hard to improve IE and the latest version of the software, IE8, is far better than earlier versions (though it still lags far behind its rivals when it comes to adhering to Web standards).

However, it’s not IE that is going to send Firefox’s market share tumbling. Nor will Apple’s Safari have much of an impact outside of the Macintosh world (other than on the iPhone, of course). And Norway’s Opera Software, which is still plugging away at its Opera browser, isn’t likely to expand its market share much beyond its current 2%.

No, the browser that looks set to knock Firefox off the number two spot — and perhaps even have a go at unseating IE further down the line — is the new kid on the block, Chrome, developed by Google in collaboration with open-source developers.

In a short time, Chrome has unseated Safari to become the third-largest computer Web browser. Its market share is still only about 5%, but the speed at which it has overtaken Opera and Safari is telling.

I’ve been using Chrome (beta version 4 for Mac) as my default browser for the past month and, despite it having a little instability, I’m on the verge of ditching Firefox for good.

Why? Well, Chrome is lightning quick next to Firefox. It starts up in an instant, and runs online applications like Gmail noticeably quicker. The software engine underlying Chrome is optimised for speed, especially when it comes to working with online apps.

It also has a cleaner interface that maximises screen real estate. It’s almost as if it’s more fun to use, too.

Ironically, Firefox has long benefited from Google’s largesse: the latter paid tens of millions of dollars to have its search engine be the default in Firefox’s search box.

But in other respects Google is not doing Firefox any favours. It has poached top Firefox engineers to work on the Chrome project.

This may already be having an impact on Firefox: the latest version, 3.6, has been delayed. Versions 3.7 and 4.0 may also slip.

The other thing that looks set to slip is Firefox’s market share. The only question, then, is by how much.