It comes off as ... I dunno ... cheap, and I don't mean in price. I know people that love it. To each his own.
Thunderbird has customization options similar to Firefox. There are a couple of extensions for Thunderbird you may want if you're accustomed to Outlook. Together they allow you to use Thunderbird with calender functions and integrate that with a Google Calender.
You need both:
Lightning for Thunderbird and
Provider for Google CalenderHere's a page with detailed instructions on setting it up to work with Google Calender.
http://bfish.xaedalus.net/2007/04/stay-in-sync-with-gcal-and-thunderbird/Ignore the Windows specific stuff.
If you'd just like the calender and don't care about synching it with Google, you only need Lightning.
Adding an extension to Thunderbird is a bit different from Firefox since you can't just click the button and have it install automatically. What you'll have to do is download the extension, save it somewhere, then go into Thunderbird, under Tools > AddOns then click the Install button and navigate to the directory where you save the extension you want to install. When I added Lightning, I had to restart Thunderbird, then go back into AddOns, click on Enable for the Lightning extension, then restart again. The second step isn't usually required. I dunno if this is an error with the packaging or just something that happened to me.
In Linux, there are a couple of system requirements for it to work. Mint has them already installed, and I imagine Ubuntu does as well. The one that may be missing is a package called libstdc++5. If you want to try this out, do this from a command line first:
sudo apt-get install libstdc++5
If it's already installed, it'll just say so. If not, it will add it, and you'll be good to go.