I want to move from Thunderbird to Outlook. Thunderbird uses MBOX and Outlook uses PST. What is the difference and can I just rename the file?

You can not rename one into the other. They are very different formats.

MBOX is a plain text file where messages are stored one after another. It is an open format and used by Thunderbird, Apple Mail and Gmail Takeout. One MBOX file normally holds one folder, so an account is a set of files.

PST is a Microsoft format. It is a single database file that holds many folders, plus calendar, contacts and tasks. It works with Outlook for Windows, has a size limit (50GB by default in current Outlook) and its own repair tool.

To go from Thunderbird to Outlook you convert. Two common ways: add the mail account as IMAP in both programs, and copy folders so they sync through the server, or use a conversion tool that reads MBOX and writes PST. If the mail lives on an IMAP server, the first way is free and safe.

My mail is IMAP so I just added it in Outlook. Everything synced. Thanks.