You have probably heard about TraceMonkey. If not, I recommend that you read these 3 excellent post about it: Brendan’s Roadmap Update, John Resig’s update and Ajaxian’s article. As pointed in the 3 articles, this is a very promissing development and will change the playing field.
If you want to experience some of that, simply install one of the latest 3.1 firefox builds and run a 1.2.x version of feedly on it.
Mozilla team: you rock! (I wish Mozilla was a public company…)