I’ve been testing IE8 for months, first using its prerelease versions and, more recently, the final version. I’ve found it to be a big improvement over its predecessor, IE7, and a much closer competitor to its main rival, Mozilla’s Firefox. IE8 is more stable than IE7, more compatible with industrywide Web standards, and packed with new features that improve navigation, search, ease of use, privacy and security.

Some of these features can’t be matched out of the box by its main rival browsers. For instance, related tabs are color-coded, the search field can show images along with text, you can get instant fly-out maps of place names in Web pages, and you can easily hide your tracks online from the prying eyes of advertisers.

But, in my tests, IE8 wasn’t as fast as Firefox, or two other notable browsers — the Windows version of Apple’s (AAPL) new Safari 4 and Google’s (GOOG) Chrome. IE8 loaded a variety of pages I tested more slowly than any of the other browsers, and it grew sluggish when juggling a large number of Web pages opened simultaneously in tabs.

For that reason, I can’t say that IE8 dethrones my previous browser champ, Firefox. 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.

The new IE8 lets you see images in results from the built-in search box and quickly switch sources.

Microsoft is making IE8 available, free, at noon EDT Thursday, for both Windows XP and Windows Vista, at microsoft.com/ie8. A version also will be tailored for the forthcoming Windows 7, the next edition of the company’s operating system. But that version won’t be available until the next prerelease iteration of Windows 7 comes out. It will also be automatically offered via the Windows Update system over the next few months.

Unlike its competitors, IE8 won’t be available in a Macintosh version, though I found it worked fine on a Mac that is running Windows alongside the Mac’s own operating system.

Favorites and Tabs

This new Internet Explorer looks a bit different, right away. It finally displays, by default, the old Links bar, now renamed the Favorites Bar. This is a toolbar near the top of the screen where you can store your most-used Web sites or folders containing groups of frequently visited sites, for convenient access. It’s like the Bookmarks Toolbar in Firefox or the Bookmarks Bar in Safari. This bar was available in older versions of IE, but was hidden unless you turned it on.

And this Favorites Bar has a couple of nice features. There’s a one-click button that will add any Web site to the bar, as opposed to adding it to the longer Favorites list of less-frequently visited sites. And, to help fit as many sites as possible on the bar, IE8 has a command that automatically condenses the titles of the entries.

There are also big changes in the way tabbed browsing works. In IE8, tabs you open from links on the same Web site are grouped together and color-coded. And when you have too many tabs to see at once, you can click on a button to see mini images of the pages they represent, or, alternatively, you can get a quick text list of all of them.

In addition, when you create a new, empty tab, IE8 displays a number of choices inside the page. These include the ability to reopen tabs you’ve closed or to perform various actions on text you’ve copied, such as emailing or blogging it.

There also is an optional Suggested Sites feature, which pops up a list of other Web pages that might be similar to, or related to, the page you’re viewing. This feature doesn’t always do a great job, but when it works, it’s handy. For example, when I was reading the BBC’s Web site and clicked Suggested Sites, IE8 listed a variety of other British news sources I hadn’t bookmarked.

Addresses and Search

Like the other major Web browsers, IE8 now also makes smart suggestions about what you might be looking for when you type something into its address bar or its search box. In the address bar, these are based on your history and your Favorites. In the search box, they are based on suggestions from whatever search engine you choose to view in the box, plus your history. All of these suggestions are organized nicely. (If you are using Windows XP, you must install Microsoft’s desktop search product for all of these features to work.)

But the IE8 search box does two cool things the other browsers don’t. First, it allows search engines to show images in the search results that drop down from the box, something Microsoft calls Visual Search. With some providers, like Google, you don’t see images, at least not today. But with others, such as Wikipedia and Amazon (AMZN), images show up.

Microsoft’s new browser IE8 includes a feature called Accelerators, which can perform specific tasks on Web pages.

Second, and more important, IE8′s search box lets you switch search providers on the fly by just clicking on an icon at the bottom of the results list. So, for instance, you could type in Red Sox, see the results in, say, Google, and then without retyping your search term, almost instantly get different results from Yahoo (YHOO) or from Microsoft’s Live Search engine, by just clicking their icons.

Surfing Tools

IE8 includes a new feature called Accelerators, which can perform specific actions on any text you select in a Web page, often without taking you to a new page. When you select text, a light-blue icon appears near it. When you click on that icon, you get a list of options. For instance, you can translate the text to another language, email it, blog it or, if it’s a place name, map it.

Depending on which company’s services your chosen accelerator is using, these actions can happen right on the page you’re viewing, in a fly-out panel. For example, I selected the word “Beijing” in a news story, chose Map with Yahoo from the Accelerator list, and got a map showing Beijing in a small window atop the same page.

When you install IE8, Microsoft suggests you use its own set of accelerators, but gives you the option to choose from Google, Yahoo and other competitors. A full list of accelerators, search engines and other add-ons for IE8 is at ieaddons.com at the bottom left of the page.

Another nice feature is called WebSlices. This requires some effort on the part of Web page publishers and is on only a small number of pages right now. But it allows a user to add to her Favorites bar a constantly updating section of a Web site, complete with graphics, by just clicking a green icon that appears on the site. For instance, I added to my Favorites bar a slice that shows the top stories on digg.com.

Speed and Stability

Microsoft claims IE8 is very fast, but in my tests, speed and performance were its worst attributes. Using two computers, one running Windows XP and one running Windows Vista, I timed the loading of a half-dozen popular Web sites, plus two folders containing numerous news and sports sites. I repeated the test in IE8, and in Firefox, Safari 4 and Chrome. In every case, IE8 loaded the pages and folders more slowly than most of the other browsers, and in most cases it came in dead last.

In some instances, the differences were tolerable — a few seconds. In others, primarily the folders containing nine or 21 sites, respectively, IE8 took two or three times as long as one or more of the other browsers to complete the task. Microsoft conducted its own tests, which show IE8 winning similar tests, but I rely on mine, which I also use when evaluating its competitors. You can judge for yourself.

IE8 never totally crashed on me. This is partly because when one tab crashes, it’s designed to leave the others unaffected. However, in my tests on both machines, I found that IE8′s general operating speed — things like opening menus or switching among tabs — slowed down noticeably when I had 15 or 20 sites opened in tabs, even after they finished loading.

Security and Privacy

By contrast, IE8 shines in the areas of protecting you on the Web. Like other browsers, it warns you when a Web site you’ve reached might be a phishing page, designed to steal your identity, or a page that’s known to distribute malicious software. And, like others, IE8 allows you to conduct a private browsing session that won’t leave any history or other evidence on your own PC.

Color-coded tabs make it easy to organize searches.

But IE8 also has a feature, called InPrivate Filtering, that the company says will optionally allow you to surf multiple Web sites without leaving the kinds of tracks on Web servers that allow advertisers and others to know where you’ve been and what you did there. I was unable to test the effectiveness of this feature, but assuming it works, it’s a step forward in privacy.

Compatibility

IE8 had good compatibility with most Web sites I visited. But in some cases, it didn’t render a page properly. This is mainly because some sites were designed for older versions of IE, which used proprietary page-rendering features that made some sites look good only in IE. With the new version, Microsoft is moving away from those proprietary features.

To solve this problem, IE8 includes a compatibility button you can click that will cause the browser to behave like older versions of IE and render the page properly. You have to click the button only once for each page, and IE8 will automatically do it for you on subsequent visits.

Bottom Line

Internet Explorer 8 is a well-done advance on an important product used by most people to surf the Web. If it were faster, I would say it was the best browser currently available for Windows. But even so, it will be an improvement for current Internet Explorer users, and might even tempt some folks to switch.

Find all of Walt Mossberg’s columns and videos online, free, at the All Things Digital Web site, walt.allthingsd.com. Email him at mossberg@wsj.com.