The latest version of SugarCRM, 7.6.0.0, breaks the search_by_module API call used by the Thunderbird extension. Sugar have released a hotfix that addresses this issue, which will be fixed in version 7.6.1.

For those affected, to request a HotFix for bug 73637, please file a case with the Sugar Support team. In the case, please provide them your current version and edition.