Microsoft will release Internet Explorer 8 to the public Thursday morning -- overhauling its widely used Web browser with new features, while trying to recapture some of the market share lost in recent years to Firefox.

The new browser is scheduled to be available for download at 9 a.m. Pacific time. New features include a built-in "visual search" tool, a private browsing mode, and a technological trick that lets users avoid manually copying and pasting text from one site to another. Microsoft is also moving more toward Web standards in the new browser, requiring a special "compatibility view" mode for some sites.

With the changes, Microsoft hopes to make Internet Explorer "a compelling thing to switch to, but also a compelling thing to stay on," said Mike Nash, Microsoft's corporate vice president for Windows product management, in an interview this afternoon.

The current version, Internet Explorer 7, was released in October 2006, after a five-year development lull that gave the open-source Mozilla Firefox browser an opening to rack up significant market share.

IE8 "Accelerator" (Click to enlarge)

Firefox has continued to gain ground in recent years. As of last month, Internet Explorer market share had slipped below 68 percent, according to Net Applications -- a 10 percentage point decline from October 2007. That's despite the fact that Internet Explorer continues to be bundled with Microsoft's dominant Windows operating system.

Internet Explorer 6 was vulnerable to Firefox in part because of security problems with that earlier version of the Microsoft browser. But many of those problems were addressed in Internet Explorer 7. So it's not clear if the improvements in Internet Explorer 8 will be enough to persuade those users to switch back, said Matt Rosoff, an analyst at the Directions on Microsoft research firm in Kirkland.

The new IE is "fast, efficient, and it works well," said Rosoff, who has been using a near-final test version. But even with the changes, he said, "it's going to be tough to make the case that you should switch back to Internet Explorer if you’ve been using Firefox."

IE8 Address Bar (Click to enlarge)

In a review posted this evening, Walter Mossberg, the Wall Street Journal's influential technology columnist, calls Internet Explorer 8 an improvement over Internet Explorer 7 and a closer competitor to Firefox.

However, he disputes Microsoft's claim last week that the new Internet Explorer is faster than its rivals.

"For that reason, I can’t say that IE8 dethrones my previous browser champ, Firefox," Mossberg writes. "If you’re a light-duty user and attracted to the new IE’s strong suite of fresh features, you might prefer it to Firefox. But if you would be bothered by the speed difference, or the slowdown I saw under a heavy load, Firefox would still be better."

Speaking this afternoon, Microsoft's Nash stood by the company's speed tests. "No other browser is as fast as IE8 is on many sites," he said. "Where we're slower, it literally is a difference of multiple blinks of the eye. It's no longer a function of whose renderer is faster. They're all about the same."

These are some of IE8's new features:

Accelerators: Web users can highlight text on a page and right-click to bring up a menu that lets them automatically use that text on other sites whose developers have taken advantage of the accelerator feature. For example, it's possible to highlight an address on a page and insert the text into an online mapping service, without manually cutting and pasting.

Web Slices: This feature lets users access selected content from Web pages through the IE8 Favorites Bar, letting them quickly see updated content. For example, someone can use an eBay Web Slice to monitor an online auction.

InPrivate Browsing: When turned on, this feature lets users keep their browsing history and other traces of their Web activity from being known to other people who use the same computer. It's known colloquially as "porn mode," but Microsoft cites other potential uses, such as buying a gift online without tipping off the recipient.

Smart Address Bar: Typing into the IE8 address bar brings up a list of addresses and sites from the user's history and favorites that match the text being typed. Like some of the other IE8 improvements, this is similar to an existing feature in Firefox 3.0. Microsoft sayd the approach will make it easier for people to find sites they've visited before.

Compatibility View: Internet Explorer has long been a laggard in Web standards, but IE8 attempts to address that by adhering to common rules for displaying Web sites. The down side is that some sites won't display properly in the new browser. Microsoft has added a "Compatibility View" button that can be clicked to render those sites properly.

Visual Search: When users enter terms into the IE8 search box, results from a selected search service -- including pictures -- can appear in a drop-down menu as they type. This lets users see the results and refine their search, if needed, without leaving the page that they're on.