Today, I upgraded my Mozilla Thunderbird email client to version 5.0. To my surprise, it turned out to be faster than the previous 4.x versions.
I was disappointed with the 4.x versions of Thunderbird. It was slower and the scrolling of the left window was not very responsive. This changed in the new version 5.0.
I strongly recommend that Thunderbird 4.x users upgrade to version 5.0. This is the best version yet of a quite reliable email client.