Firefox is one very hot browser, pardon the pun, and your computer will be safer if you switch to this free software that’s heating up the charts.

There’s been a seismic shift during November, with Firefox emerging as the first credible competitive threat in a decade to Microsoft’s aging Internet Explorer browser for Windows.

The rush started on Nov. 9, when the non-profit Mozilla Foundation, based in Mountain View, Calif., released the finished 1.0 version of Firefox after many months of pre-release “beta” development.

Millions of Internet users have since installed Firefox 1.0, which is available as a free download. OneStat.com, a Web research firm based in Amsterdam, said last week that Firefox and related browsers from the Mozilla Foundation now account for 7 percent of global Internet usage – up from just 2 percent in May.

Internet Explorer’s market share has dropped by an equal amount, to 89 percent from 94 percent.

There are two reasons why you should care about this:

First, Internet Explorer’s outdated design is full of security flaws, despite some major patching in the recent Windows XP Service Pack 2 upgrade. IE’s huge market share also makes the browser a major target for cyber-criminals.

Second, Microsoft has been asleep at the switch in adding features and design improvements. While the current version of IE was a state-of-the-art browser when released three years ago, Firefox has now pulled ahead with several clever improvements.

Switching to Firefox is surprisingly easy. You start by downloading the 4.7-megabyte install file from Mozilla’s Web site (www.mozilla.org/firefox). It’s available for every version of Windows back to Windows 98, as well as the Macintosh and Linux. Mac users, however, are best served by Apple’s own Safari browser.

Then you run the install file, which takes little more than a minute.

If you follow the default choices when you first open Firefox – and you should – the browser will import your favorites, also known as bookmarks, from IE, along with other stored information such as the addresses of sites you’ve recently visited. Firefox will also become your default browser, meaning that Firefox will open instead of IE whenever you click a Web link in an e-mail or other document.

The first thing I noticed after launching Firefox was how little I noticed.

Familiar menus and control buttons from IE are found in the same positions along the top of the Firefox screen, altered only slightly. The “Forward” and “Back” buttons, for example, are green arrows in Firefox, instead of IE’s green circles with white arrows inside. The “Home” button in both browsers is a white house with a yellow roof, the only difference is that IE’s house has a tiny chimney.

Normal people outside the tech community – who don’t obsess about browser market share, the future of open-source vs. commercial software and similar arcane subjects – would hardly notice the switch from IE to Firefox. The learning curve is zero, unless you want to take the optional step of mastering the extra features missing from IE.

Those extra features certainly are worth exploring, especially for power users.

The most important is tabbed browsing. You can open multiple pages within a single browser window in Firefox, with each page becoming a file-folder-like tab across the top of the screen. To move from one page to another, you just click on the tabs. This is much faster and more efficient way to cope with multiple pages than opening separate browser windows in IE, littering incomprehensible icons along the Windows taskbar.

Firefox also has a nifty integrated search box in the upper right corner of the screen. You can type a search term into the box and get an immediate Google search, without going to Google’s Web site. You can click the Google icon in the box for a pull-down menu of other search sites, such as Yahoo and Amazon, and can even add search sites of your own choosing.

There’s more, which I don’t have room to cover here, making Firefox a kind of playground for Net geeks. Firefox also is slightly faster than IE in loading pages.

But the real reason to switch is security. I called three experts last week: R. Scott Granneman, a computer security consultant in St. Louis who’s also writing a Firefox how-to book; Eric T. Peterson, a Web analyst in San Diego for the firm JupiterResearch; and Johannes B. Ullrich, chief technical officer of the SANS Internet Storm Center security service in Quincy, Mass. All three recommended that home computer users running Windows make the shift to Firefox.

That’s because the Internet is swarming with viruses, worms, Trojans, spyware and phishing traps. IE is a conveyor for some of these maladies, while Firefox so far is falling below the bad guys’ radar. Another non-IE alternative is the free Opera browser (www.opera.com), which I reviewed favorably in March.

I’ll close with two very important pieces of advice:

Firefox is not a cure for cancer. You still need to take all the burdensome steps required for safe computing: installing anti-virus software and keeping it up to date, using firewall software if you have a broadband connection, running an anti-spyware program, and making sure not to fall for phishing e-mail that tries to talk you out of personal information. If your computer runs Windows XP, you should also upgrade to Microsoft’s free Service Pack 2, despite the very slight risk it will hurt your system.

Also, you don’t have to give up on IE entirely. Some Web sites require IE for certain functions, or just look better with IE. There’s no big security risk in using IE to visit sites you know and trust, as long as Firefox is fixed as your default browser.