I recently switched from Opera to FF (beta channel) because Opera now working on webkit based browser and the current Presto-browser doesn't get any dev love beside security updates. One thing I noticed is the really slow start up time of FF and high memory consumption. Also I have the feeling it gets worse over time because when I start a new profile (even with the same addons) FF feels suddenly much more responsive, starts faster and so on. I hope this stuff can help them improve those issues.