"With this technique, Opera is compiled twice. The first compilation creates a specially instrumented build that gathers statistics about which code is called and which variables are used most often. The instrumented build loads sites automatically using “URL player”, a testing feature available in every Opera build (more information about that in a later blog post). This information is dumped into files and used to produce the final build that’s optimized based on the gathered statistics."
Be sure to checkout our more complete article where we compare the performance of more browsers.
I am still an firefox fan and i know firefox will come up with something like this with their next release
btw firefox have released an alpha version of firefox 4, testers haven't noticed any change, i think they are probably testing the engine