Nothing of importance. A combination of laziness and not liking the appearance of buttons in 2.0. I do have 2.0 on one of my other machines, just not the one I usually use. I just mentioned the version in case anyone else having trouble with the site was still on 1.5.
Cool. Browsers are a lot about personal preferences. I use the Noia Extreme skin, so I don't really worry much aboout the default appearance of FF or Thunderbird. Once they stop releasing patches for the 1.5 line, I'd recommend upgrading though.