Two of the four major browsers have undergone some big changes in the past two weeks. Firefox 3 is, of course, the big news of the week, pulling down eight million or so downloads in its first 24 hours in the wild. However, the Opera browser updated to its much-awaited version 9.5 last week. Since both of them have got game but for different reasons, let's take a look at how they match up.

Empirically, the two most-cited complaints about browsers are speed and memory. Now, I'm a big fan of Firefox because it's so easy to customize, so despite concerns I had about placing both browsers on "equal footing," it would be misleading to test Firefox devoid of extensions, so I left in my cadre of add-ons, and ran both with fifteen tabs open--a more or less standard browsing session for me.

Opera's browsing Notes. (Credit: CNET Networks)

Using the SunSpider JavaScript test, Firefox 3 scored around 5500 microseconds to process the tested scripts, with a margin of error at around three percent. Opera 9.5 scored about 7280 milliseconds on the same test, with a margin of error around 1.5 percent, making it nearly one and a third times as slow as Firefox 3. This isn't surprising, given how long Opera's been using the same rendering engine. Assuming they get to Opera 10 before Mozilla puts out Firefox 4, it would be interesting to go back and re-compare them.

Firefox used approximately 127 MB RAM with the 15 tabs open, while Opera used around 117 MB. This was a little bit surprising, since Opera is definitely running on the older code. It might be attributable to the various extensions I use in Firefox, but it's more likely that despite the claimed 15,000 fixes deployed in Firefox 3, there's still quite a bit of room to plug those memory leaks.

Firefox 3's "Awesome bar". (Credit: CNET Networks)

There's more to browsing than just benchmarks, of course. Users who browse by cell phone and like to keep all their personal settings synchronized are sure to look favorably at Opera, which has made a name for itself breaking into that market. Opera also bakes in a lot of excellent features, from its location bar--most likely the inspiration for Firefox's new "awesome bar"--to speed dialing, password management to resumable downloads, an icon-based sidebar to a synchronizable notepad.

Firefox, of course, has its plug-ins, and that level of user innovation and customization built on an open-source foundation clearly has been a winning combo for Mozilla. Many of the newest features in Opera 9.5, like the ability to see if a password works before saving it, are also part of Firefox 3. Pausable downloads, session management, and searchable bookmarks are also available in both.

Choosing between these two alternative has more to do with personal preference and browsing style than either one sporting a killer feature. With Mozilla claiming more than 20 percent of the browser market between Firefox 2 and 3, though, it's obvious there's only one alterna-juggernaut.

UPDATED: Corrected "microseconds" to milliseconds.