Weird. It's super smooth even in the Android WebKit control embedded in the HN app on Android 4.4.2. It's been a long time since I switched to Chromium but I really thought Firefox got faster by now.

Quick screencast: https://docs.google.com/file/d/0B4MGwWE_SNv0elNfUVdmalp5NVE

What version/OS are you on?

I'm running 29 (Aurora? Beta? Whatever it's called :)) on OS X and it's perfectly smooth, low CPU usage.

I've got the same problem with version 28.0 on Linux.

28, Linux