How to migrate emails and contacts from Outlook (desktop version) to Thunderbird on iMac?

For messages, if you have an IMAP account then it's easy. You just make sure that all the messages in Outlook are in folders that synchronise with the server; then you create the account in Thunderbird and it downloads all the messages and folders from the server. You can continue to use both Outlook and Thunderbird if you want since they both derive message data from the same server account. For contacts, it's fairly easy although it might involve a little work. Outlook can export your contacts in standard .csv file format. You can import that into the Thunderbird address book. The import will go smoothly if the names of fields and their contents match what Thunderbird expects. The Thunderbird import wizard allows you to match the fields to some extent, but it's sometimes easier to open the .csv file first in a spreadsheet program and edit the data manually before importing it.