Set one master password to keep the rest of your saved passwords locked up tight.

Apple’s Safari is a fine Web browser, but many prefer the open-source, cross-platform Firefox, which lets you customize it to heaven and back with themes and add-ons, called extensions, that give more functionality. And the brand-new Firefox 3 packs a ton of improvements, boosts performance, and still comes in for the low, low price of nothing. (What a country!)

Interface changes jump out first. The Back button is extra-large (to shrink it, right-click the toolbar, choose Customize, and select Use Small Icons), and the new Smart Address Bar has a blue star button that instantly bookmarks the current page.

Speaking of bookmarks, Firefox 3 changes for the better what you can do with them. Click the Bookmark star again (or press Command-D) for a semitransparent floating window where you can add tags and/or file the bookmark in a folder or the Bookmarks Toolbar. You can add a dedicated keyword to go directly to a specific bookmark, but unfortunately, that option is buried in the Bookmarks Organizer. We would’ve loved to see it in the bookmark pane with the tags. When you start typing in the Smart Address Bar, FF3 generates a list of possible results by searching your bookmarks and their tags, as well as the URLs and page names in your browsing history. The letters you type are highlighted in the results list, and you can use the arrow and Return keys to navigate the results and launch pages.

The Downloads window (Tools > Downloads, or Command-J) is also improved, letting you pause and resume downloads, and search for files you downloaded previously. The Add-ons menu (Tools > Add-ons) now lets you find new add-ons (extensions and themes) right in the window, instead of just managing ones you’ve already added. You can zoom images and text, or one or the other, and FF3 remembers your zoom level on pages you’ve visited before. Speaking of remembering, the new browser will still restore your last session after a crash, plus you can now save all your open windows and tabs anytime you quit.

You can set a master password (which you’re prompted to enter once per session) that unlocks your saved passwords. And when you enter a password on a webpage, you won’t see Firefox 2’s obtrusive pop-up dialog, which forced you to click Save Password, Not Now, or Never For This Site before it would let you move on. In Firefox 3, a more subtle gray bar gives the same choices, but below it, the next page in your session loads—which means you can see if that password actually works before you tell Firefox to go ahead and save it. (Or ignore the bar altogether.)

Speed is improved thanks to a new engine and better memory management—we saw zippier performance in our testing, especially in Web 2.0 apps like Google Mail and Zoho Office. (Another cool feature: In Preferences > 
Applications, you can set Web apps to handle standard protocols, such as having Google Mail launch when you click a “mailto:” link.) The Gecko 1.9 engine boasts improved rendering of fonts, graphics, and animations. Browser speed and performance vary widely depending on what you view and how many tabs you keep open, but overall we noticed a performance boost on our 2.4GHz Core 2 Duo MacBook Pro.

If your favorite Firefox 2 extension hasn’t been updated to work with Firefox 3 (check Mozilla product manager Alex Polvi’s blog at alex.polvi.net for the latest compatibility report, but as of June 26, 80 percent of FF2 extensions could play nice with FF3), don’t fret. MultiFirefox (free, codecontortionist.com) lets you keep your Firefox 2 profile intact while starting a new one for Firefox 3 and even lets you run both browsers alongside each other. So if your favorite FF2 extensions aren’t compatible with the new browser yet, you can still try out FF3 and decide later which to stick with. (MultiFirefox came in very, very handy when we wrote this review, so a huge thanks to its creator, Dave Martorana.)