The upgrades are the latest skirmish in the browser war that started in the mid-1990s and led to Microsoft's triumph over Netscape. The battles reignited in 2004, when Mozilla's Firefox launched and revealed new avenues of development.

On Tuesday, Opera Software ASA is releasing its Opera 9 browser, while Microsoft's Internet Explorer and Firefox are in line for major overhauls later this year.

The most anticipated update comes from Microsoft Corp., whose 5-year-old, market-leading Internet Explorer 6 browser, or IE6, shows signs of aging.

The software company, which has seen IE slowly losing market share to Firefox, hopes version 7 will bring the browser to parity with its rivals, while adding features to thwart "phishing" scams and make browsing more secure.

"IE6 was easily the best browser available in 2001," said Dean Hachamovitch, Microsoft's general manager for IE. "The challenge is people use the Web a lot of differently now. Search engine usage, there's a lot more of that now. Safety, there's a lot more malicious intent on the Web right now."

Today, e-mail, maps, word processing and other traditionally standalone applications are migrating online. Major Internet companies such as Google Inc., Yahoo Inc. and even Microsoft are devoting tremendous resources developing these Web applications - and browser developers want them to run well.

Opera 9 sports "widgets" - Web-based applications that run off its browser but appear detached as standalone tools. Anyone knowing Web coding can develop widgets for Opera to check weather, soccer results or the status of eBay Inc. auctions; others can download existing ones.

"Most end-user applications being developed today have at least part of their functionality running on the browser, which is completely different from the way it used to be 10 or 15 years ago," said Christen Krogh, Opera's vice president of engineering. "In the old days, browsers were like printing presses" - displays for static pages.

The new Opera, making its debut in Seattle to invoke images of Opera Chief Executive Jon S. von Tetzchner landing in Redmond, Wash.-based Microsoft's backyard, also formally supports a file-sharing mechanism called BitTorrent and lets users customize preferences - such as whether to allow JavaScript - on a site-by-site basis.

With version 7, IE will have its first search box in which users could type queries without visiting a search engine's home page. Firefox and Opera have long had that feature in response to the growing use of search engines to find Web sites.

IE7 also will join its rivals in supporting domain names that use non-English characters.

And it will play catch-up by sporting tabbed browsing - the ability to open several Web pages at once without creating separate windows. Although Opera and Firefox have had it for years, Hachamovitch said IE7 will go further with Quick Tabs, in which users can view small, thumbnail versions of all open pages at a glance.

Hachamovitch also said IE, a frequent target of hackers, will in version 7 go beyond the security enhancements IE6 received in 2004 as part of the Windows XP Service Pack 2 upgrade.

A version shipping with Vista computers, due out for consumers early next year, will come with parental controls and a "protected mode" so hackers can't easily to gain access to the rest of the machine even if the browser is hit.

The regular version, scheduled to leave the "beta" test phase in the second half of the year, will block or warn about scam sites, while its address bar will turn green when an e-commerce site has gone through additional background checks to receive a so-called high-assurance digital certificate.

Firefox 2, a "beta" version for which is planned this summer and a full version by September, will also include anti-phishing features, along with tools to automatically restore Web pages should the browser suddenly crash or require a restart. Other features in the Mozilla browser include a search box that can suggest queries as users type.

And Mozilla already has its sights on Firefox 3 next year, with plans to let users run online applications even when there is no live Internet connection.

Meanwhile, Flock Inc. released last week a test version of its Firefox-based Flock browser. Tapping into the recent wave of sites that encourage users to share content, Flock makes it easy to drag and drop images to MySpace.com and automatically notifies users when friends add items to selected photo sites.

IE7 will require later versions of Windows, including Service Pack 2 of XP, while Opera, Firefox and Flock will run on Macintosh, Linux and older Windows machines as well.

Already, IE has seen its U.S. market share on Windows computers drop to 90 percent from 97 percent two years ago, according to tracking by WebSideStory. Firefox's share has steadily increased to 9 percent, with Opera's negligible despite its innovations.

WebSideStory analyst Geoff Johnston said Firefox must continue to improve just to maintain its share. Because IE automatically ships with Windows, he said, users satisfied with IE7 may not find enough reasons to download and install Firefox when they buy a new computer.

"It takes a lot of energy to switch technology," Johnston said. "You really have to care. It comes down to the `If it ain't broke, don't fix it' mentality."