Just as was the change in Firefox 7.0 Nightly, the entry extensions.checkCompatibility.7.0a will not work with the new Thunderbird 7 nightly. To disable the compatibility checking add the following Boolean entry and set it to false:
extensions.checkCompatibility.nightly
This only works for Thunderbird 7 Nightly builds.
This can be solved using: https://addons.mozilla.org/en-US/thunderbird/addon/add-on-compatibility-reporter/