Re: outlook mail *.pst file
PST and MBOX both are different file formats. In case, you want to access existing emails of MS Outlook emails in Thunderbird then conversion of the data file format is compulsory.
I agree with 'Antikythera' for guiding the manual way to complete this task. But, it may be tedious for non-technical users.
Last month, I did this migration with the assistance of third party software i.e. Stellar PST to MBOX Converter Software. However, it is a paid software but available with free demo.