Install the "Add-on Compatibility Reporter" extension, and you can install extensions that have not been upgraded yet. Zindus works for me this way. (I haven't tried Virtual Identity.)
EDIT: For Zindus, the version you add is for the older Thunderbird, but if you check for updates after installing it, it will upgrade the extension to the latest version, which is compatible with Thunderbird 5. I'm not sure why it does not simply add the latest version by default.
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