Browser speed comparisons if quite often subjective, and very dependent upon external factors:
- Safari enjoys a lot of speed gains on Mac OS X because it uses undocumented APIs; these APIs are not stable, thus the only way to really gain from them is to keep the browser in flux. The Mozilla team is at a disadvantage here because it aims at delivering a stable browser that doesn't rely on tricks; moreover, Firefox releases are extensively tested by the whole community, while Safari is tested by... Apple. Which delivered a heavily unsafe browser release no later than last week. A 'good' way to compare the browsers is by running both on a 'neutral' platform: Windows (nice, as it puts IE8 to shame), or Linux (where both development teams have access to all APIs indiscriminately).
- Gecko 1.9 is still mostly backward compatible with 2001's Gecko 1.0 release. Several internal APIs have been kept for compatibility reasons, but slow down code execution. The 'real' speed gains (in Javascript) will probably be found in Firefox 4/Gecko 2 (as the article mentioned, but didn't expand upon) with Tamarin, and it will be cumulative with the next point.
- Firefox uses Gecko for its whole UI; this helps against code branching, because the same javascript,CSS etc. files are used for a UI element on win32, OS X or X11, but it does slow things down. Safari can be found on Mac OS X, and is ported to Windows. To overgeneralize, Firefox is a platform-agnostic application running on its Gecko toolkit (so, if Gecko gets faster, Firefox gets faster too), Safari has native ports.
As for Opera, well, it is... Special. Don't take it negatively, but it actually is the odd one out there. Yes, it's fast, stable, and filled with nice features; it's standards-compliant; it's available on multiple platforms. Yet, you hardly can follow its development (it's better than IE, sure; compared with Minefield and Webkit nightly releases, though...) and it's not big enough to shake things with its not too frequent releases. It also lacks powerful developer tools (Firebug, which is a deal breaker for Firefox, but also the Netscape legacy), doesn't have a shiny UI (where Safari, well, shines) and is distributed with... er... nothing (IE keeps existing because of Windows, Safari is force-fed through iTunes and Mac OS X, Firefox has a community of enthusiasts).