Your problem is most likely that you installed Thunderbird & Enigmail on Linux from the Ubuntu repositories. This means that Enigmail is installed globally on Linux. On Windows, you'll have Enigmail installed in your profile, such that you end up with two different versions of Enigmail on Linux. I assume that you have a 64-bit version of Ubuntu installed, otherwise this wouldn't be a problem.
In theory you have 2 solutions:
- Use a version of Enigmail that contains binary libraries for all platforms
- Uninstall Enigmail on Windows and install it globally
Solution 1 is only possible if you want to hack around the XPI, so I'd say the only option for the moment is 2.
To install Enigmail globally:
- Uninstall Enigmail from the Addons manager and exit Thunderbird
- Locate the "extensions" directory in your Thunderbird installation folder (e.g. C:\Program Files\Mozilla Thunderbird\extensions)
- Copy the Enigmail XPI file into the extensions directory.
- Start Thunderbird. You'll be asked if you want to install Enigmail gobally.
See also the corresponding aricle on MozillaZine