Thunderbird gained 130,000 users as part of a merger of two French tax authorities: The Directorate General Tax (DGI) and the Directorate General of Public Accountancy (DGCP). They have switched to Thunderbird because they didn't want to renew the licenses of the old proprietary software which were Lotus Notes and Microsoft Outlook. 

They also added two components to make Thunderbird meets their needs : Mozilla's Lightning to provide calendar support and OBM (from French open source development and services company) to provide groupware and collaboration tools.

OBM collaborative software

OBM has been deployed with a number of government authorities in France, with 110,000 users in the Department of the Interior, 8,000 users in the Ministry of Culture and 3,000 users in the National Assembly.