https://support.mozilla.org/en-US/kb/switching-thunderbird#w_importing-windows-mail-contacts Tools > Import Select 'Address Books' and click on next If you have 'Outlook' currently running as default mail client and you want to try a direct import - not always successful... select: Outlook' and click on Next If you have exported the Outlook contacts as a .csv (comma separted,csv) file then do this: Select: 'text file (LDIF, .tab, .csv, .txt) and click on Next a new window opens locate the folder and .csv file (you may need to make sure the 'comma separated .csv' option as file type is selected to show those files- I'm using Vista and it defaults to LDIF.) Click on Open You may need to map the fields so they correspond with Thunderbird fields.