Yes - been using Thunderbird for quite a while now. I used to use Outlook (not Express) when I needed the Calendar/Diary function. Can't really say I've noticed any difference with Thunderbird. Firefox has got so its very slow to start first time, but I usually use that, although its from habit as I find Google's Chrome to be much quicker. I only ever use IE for those sites that don't work properly in Firefox.
Barrie