As before, Internet Explorer isn't included since we're on Linux here. Opera isn't open source despite being free so it's not participating here, too. I'll just look at Firefox, Chromium, and Epiphany.
As the web moves towards broad adoption of HTML5 the scores seen here will gain in importance. Chromium (hence Chrome?) leads here with Firefox 17 not far behind. Firefox has improved a lot since Firefox 4 where Mozilla's offering lost to even Epiphany.
All 3 browsers scored full marks here. Acid3 is a pretty old test by now. I'd be surprised if any current day browser can't score full marks here.
Conclusion, nothing too interesting to see here.
With all the hype about how Chrome is the fastest browser out there, it's perhaps quite a surprise that Epiphany got the best score here. While a single benchmark result isn't conclusive by itself, this still means the major browsers aren't the only innovators in the browser space.
Firefox doesn't win it's own benchmark. Of course, that's not really a surprise.
On the other hand, Chromium does win it's own benchmark. Even Epiphany does better than Firefox in V8 by a significant margin.
If, like me, you hardly use anything other than the default browser (no plugins) then Epiphany turns out to be a pretty darn good browser. Better than Firefox in some cases.
In terms of plugin support, Firefox is still king. Chromium still holds the performance crown even if it does surprisingly lose out to Epiphany in SunSpider.
Even it Firefox isn't the best browser out there, the fact that it's Ubuntu's default browser, has excellent plugin support, and is still a browser with good performance means the majority of users probably won't be motivated enough to look elsewhere for their browsing needs.
But at least now you know where the browsers stand.