I assume you mean Ubuntu 9.10?
With Thunderbird you should just be able to find your Thunderbird profile, make a backup of that profile and then get Thunderbird in Ubuntu 10.4 to point to the copy of the profile.
1) To find out where Thunderbird is keeping your original profile go to "Tools", then "Account Settings". Select "Local Folders" and in the "Local directory:" text box you should see the location of your profile.
2) Find this location and copy the folder to a backup location.
3) Copy the backed up profile from step 2 to a location suitable under your new Ubuntu.
4) As in step 1, find the profile location text box in the Thunderbird installed on your new Ubuntu.
5) Paste in the location of where you copied the profile.
6) Save the settings (by just pressing "OK") and you should be good to go. You might have to restart Thunderbird.
Caveat: Make sure you backup all your data first and keep a note of where all the original profiles are kept just in case.