(I apologize, Opera fans, for leaving Opera 9 out of this discussion-and promise I’ll come back to it before too long.)

For the first time since the bygone days when IE 4 battled with Firefox granddaddy Netscape Navigator 4, we have a real battle. (For years, IE didn’t face truly serious competition–then Firefox 1.0 came along, and suddenly there was no question that it was the best browser for most people.)

In Internet Explorer 7, we have a Microsoft browser that has no glaring deficiencies–which doesn’t sound like much of a compliment, but it’s more than you could say about IE 6. But here’s a compliment that really is complimentary: It’s inched ahead of Firefox in a few respects. It’s got a better printing engine. Its zoom feature lets you resize Web pages onscreen, while Firefox can only resize text, not graphics. And I like how IE now lets you see thumbnails of all the pages in all your open tabs at once. I wish Firefox 2 had all these items built in.

But the funny thing is, Firefox 1.5 already has some of these features–and dozens more that neither Microsoft nor Mozilla has gotten around to implementing–courtesy of the array of amazing extensions that let you customize the browser to a fare-thee-well. (My current fave: Google Browser Sync, which lets you turn all your copies of Firefox on all your computers into clones of each other with the same bookmarks, cookies, and other settings. It alone is a compelling argument for choosing Firefox over IE 7.)

While some extensions aren’t yet compatible with Firefox 2, they’re the single biggest reason to opt for Firefox over IE 7. There are IE 7 add-ins out there, too, and Microsoft is cultivating the development of more. But for now, Firefox is easily the more vibrant development platform, and it does all its good stuff not only in Windows, but in OS X and Linux, too.

As my Computerworld colleague Scot Finnie says in his Firefox 2 first look, the Mozilla upgrade is no giant step beyond Firefox 1.5. Is it a must for 1.5 users? Yeah, as long as you’re not dependent on any extensions that don’t work yet. Features like search suggestions that appear as you type in the search box, spell checking, better RSS support, and additional tab-related conveniences aren’t life-changing, but I use most of ‘em almost every day, and they all make my time online more productive and happy.

Which isn’t, sad to say, true of every change in IE 7. Why did Microsoft turn off menus by default (without replicating all of their functions with toolbars), put the menu bar under the address window even when you do turn it on, and move the refresh button from its traditional location to the left of the address bar over to the right? I’ve asked multiple Microsoft executives about these changes, and the answers, which have ranged from, essentially, “I can’t remember” to “For consistency with Windows Vista,” have never been entirely satisfactory.

Change for the sake of change is par for the course with a Microsoft upgrade. And there’s a fair amount of it in IE 7, while things that cry out for fixing, like the browser’s convoluted configuration settings, haven’t gotten the attention they deserve.

(Side note: If you use IE 6 and decide that you’d prefer for menus to live under the address bar, it’s easy to drag them there. But this customization feature, along with other interface-tweaking options, is gone in IE 7; there’s no way to move the menus back to where they sit in 99.9% of the planet’s other applications.)

So do I have a personal answer to the question “What’s the best browser?” Ultimately, I agree with Erik’s take: Overall, Firefox maintains a lead, even though it’s not gigantic and isn’t there in every aspect of the browser. Like Erik, I think it’s a good idea for anyone who’s moving from IE 6 to try both IE 7 and Firefox 2.

And I’m still fascinated by one question: If Firefox had never cut sharply into IE’s market share, would Microsoft ever have gotten around to bringing its browser into the modern age?