How Thunderbird and Firefox find their configuration files
Thunderbird looks into the binary %APPDATA%\Thunderbird\profiles.ini file for its "registry", which contains, amongst other information, a pointer to the directory where the profile is stored (usually located in %APPDATA%\Thunderbird\Profiles\profileName).
Likewise, Firefox uses a similar file, located in %APPDATA%\Mozilla\Firefox\profiles.ini.
The value contained in this key is a litteral value, no variables (such as %USERPROFILE%/MozProfile) allowed. We chose to point this to the user's home directory (H:\Thunderbird).
The configuration itself is stored in the specified directory, in a file named prefs.js. This is a readable file of javascript commands. All other files in that directory are dynamically created by Mozilla on first start.
Unlike Mozilla's binary registry.dat file, Thunderbird's and Firefox' profiles.ini file is a readable text file. Here is an example of what this may look like:
[General] StartWithLastProfile=1 [Profile0] Name=default IsRelative=1 Path=Profiles/default.uda
If you want to point this to a location of your choice (for example H:\Thunderbird), you need to perform to changes:
Set IsRelative to be 0 Update the path to point to the desired place. Note: For absolute paths Thunderbird uses Dos'ish backslashes (\) rather than Unixish slashes (/).
You'd obtain a file such as the following:
[General] StartWithLastProfile=1 [Profile0] Name=default IsRelative=0 Path=H:\Thunderbird
A discussion about this file can be found here