Using SunSpider, a webpage that puts the latest browsers through their paces, I came out with the following results on a current MacBook Pro.
The time represents how long it takes each browser to complete a complex set of tasks.
Safari slightly edges out Firefox, but not by much. Moving to the very latest, bleeding edge versions of these browsers, you can see just how much we have to look forward to in terms of speed increases in the weeks/months ahead. The differences are startling.
These releases are pretty stable, and if you want to try them out, you can find downloads for the latest nightly builds of Firefox here, and Safari here. You can install both without overwriting or affecting your existing version.
(Thanks to @fredericl on Twitter for providing the inspiration for this article)