With its use of tabs and the ability to change its appearance, the Mozilla Firefox Web browser has taken a bite out of Microsoft's Internet Explorer market share. But what really sets it apart is a collection of unique and useful extensions.

Extensions are programs that add features and functions to the original browser. Once installed, some extensions appear in your toolbar. Others are hidden until you right-click in the browser. This lets you have dozens of extensions without cluttering your browser.

To find extensions, open Firefox and click Tools, Extensions. Click Get More Extensions at the bottom of the dialog box. This opens up a new browser window.

You can search for an extension by newest, most popular, highest-rated and category. Each listing provides a brief description, ratings and comments by users and a link to download the extension. To install, click the download link. A dialog box will appear, prompting you to install the extension. In most cases, you must close Firefox and reopen it to make the extension active.

There are more than 800 extensions available. Here are six of the best. Their links are on my Web site at www.komando .com/firefox.html.

1. Quickly finding the definition of a word or phrase is a snap with Answers. Left-click your mouse and depress the Alt key (Option key on a Mac) over any word. Facts and definitions aggregated from several sources, including dictionaries and encyclopedias, will appear. If you want to open an answer in a new tab, left-click your mouse while simultaneously depressing the Ctrl and Alt keys. For phrases, highlight them with your mouse, right-click and select "Look up on Answers.com."

2. Forecastfox displays current and future weather conditions. Once installed, click Tools, Extensions and double-click Forecastfox. This opens the options dialog box. Here you set the forecast location and where and how you want the forecast displayed. Forecastfox allows you to create and switch between different international locales. Very useful for travelers with laptops.

3. Gmail Manager and Yahoo Mail Notifier monitor your Gmail or Yahoo e-mail accounts and notify you whenever a new message arrives in your inbox. Both extensions give you a variety of ways to notify you of new mail. You can set them to display an icon on the status bar or alert you through a pop-up window. They'll also play a sound alert.

4. Not all Web sites will display properly in Firefox. When that happens, use IE Tab to launch Internet Explorer within Firefox. Just right-click on a page and select "View this page in IE Tab." Or, click on the Firefox icon in the status bar. The extension was intended for Web developers. But it's very useful for when a Web site doesn't look quite right.

5. Viewing small pictures on a Web page can be hard on the eyes. Image Zoom adds a zoom feature. Simply right-click on an image to access the zoom menu and choose from preset zoom sizes. If you zoom in too much, the image will distort. But zoom can be very useful.

6. If you're looking to kill some time, download and install Cards. It provides access to more than 30 solitaire games, such as Canfield, Klondike and TriPeaks. To access the games, click Tools, Cards.

If you no longer find an extension useful, it can be easily uninstalled. Click Tools, Extensions. Click on the extension you no longer want, then click Uninstall. Restart Firefox to finish the removal.

Contact Kim Komando at gnstech@gns.gannett.com.