Thunderbird 18.104.22.168 that was released a few days ago is finally a universal binary on Macintosh. Although the PowerPC version wasn't running that bad in emulated mode compared to what it was on the PowerPook, the native Intel binary is *fast*! Considering how much I use Thunderbird throughout the day, this is great!
Don't trust the automatic updater that kindly asks you to restart Thunderbird to have the new version: it upgrades the PowerPC binary! You have to download a brand new full version.