All of the above is useful, but . . .

I'm pretty sure the original problem (install thunderbird) can be accomplished by doing the following at the command prompt:

apt-get update

(This updates the list of available packages)

apt-get install mozilla-thunderbird

(This installs thunderbird)

The "getting headers" snafus were probably some little Internet thunderstorm or another -- either a debian ftp site was down or the original poster had a buggy connection that time with his/her ISP. I would advise trying again with the two commands above before trying anything more elaborate (unless you're in it for the learning!).