We at INFORBIRO - information technology agency as Free Software advocates use Thunderbird as our mail client. It is very robust, has great capabilities, supports many different email accounts and it's easy to migrate between different environments!
Our developers, designers and consultants often need to use more than one computer (desktops, laptops, netbooks...) and it is a priority for them to have mail access from all machines. So I am sure that the instructions will be very useful to you as well.
In short, migration a Thunderbird profle is a copying of the profile folder from one environment to another and editing profiles.ini file.
For example, here are steps for migrating a Mozilla Thunderbird profile from Windows 7 to Ubuntu 11.10:
1) Thunderbird is already default mail client in Ubuntu 11.10 so you do not need to install it manually
2) Close Thunderbirds if they are started on both environment
3) In Ubuntu go to a thunderbird profile folder, e.g. \home\%user_name%\.thunderbird\etw22loq.default (replace %user name% with your user name)
If you can't see .thunderbird folder it means that is hidden, so press Ctrl+H to show all files and folders.
4) Delete all files from profile folder (make a backup of the folder first!)
5) Open profiles.ini file in a text editor
6) Check if there is a line with your profile folder name, e.g. Path=etw22loq.default. If it contains other path replace it with a valid one.
Example of profiles.ini in GNOME on Ubuntu environment:
7) Go to a profile folder in Windows 7, e.g. %drive%:\Users\%user name%\AppData\Roaming\Thunderbird\Profiles\hje43myq.default (replace %drive% with a proper drive letter and %user name% with your user name)
If you can't see AppData folder that means it is hidden. To show hidden files and folders in Windows Explorer: Organize > Folder and search options > View > Show hidden files, folders and drives
8) Copy all files from the Windows 7 profile folder into the profile folder in Ubuntu
9) Start Thunderbird and you should be able to see all migrated email accounts from Windows 7 on Ubuntu.
If you need to migrate an Thunderbird profile from Linux to Windows consider that on Windows 7 file profiles.ini is located in %drive%:\Users\%user name%\AppData\Roaming\Thunderbird\ (not in folder containing profiles folders!). In most GNU/Linux GNOME environment it is in the same folder as profiles are.