The first major upgrade of Firefox has been released in beta form.

Version 1.5 contains improvements in performance, extensibility, security and privacy, according to Mozilla, and is the first major upgrade of the open source browser since it was launched in November last year.

Codenamed Deer Park, Firefox 1.5 will have a second beta version, planned for 5 October, and then possibly two or three follow-up releases, starting in late October. A final version will most likely be launched in November or December, said Chris Beard, head of products and marketing at Mozilla.

The beta releases are primarily aimed at software developers for testing and feedback, but anyone interested can download the software, Beard said. The follow-up versions are closer to what the finished product will be.

After years of absolute dominance of the browser market by Internet Explorer, Firefox has emerged as a significant alternative with between seven percent and eight percent of the market.

"We've had a pretty successful year following the launch of Firefox 1.0 and we're continuing to see very strong demand," Beard said. Mozilla estimates there are between 40 million and 50 million active Firefox users, defined as those who use the browser on a daily basis, and the browser is downloaded hundreds of thousands of times every day, he said.

Although Firefox's adopters are predominantly technically savvy users, mainstream adoption has been increasing, and version 1.5 is expected to help further the browser's popularity among the non-technical, Beard said.

Beta 1 of Firefox 1.5 will offer faster website navigation, better tabbed browsing feature, RSS support and an automated update feature.

"We're delivering a software update capability that is holistic, that takes into account how people use Firefox," Beard said. "This feature is one of the most significant enhancements. It has been a pain point for Firefox users and we believe we've delivered a solution that will let people stay current."