Thunderbird reads the MBOX format natively, so this is the free route if you are fine with installing an email client. On a fresh install it asks you to set up an email account first, but you can skip it because Local Folders is available.

• In Thunderbird, open Account Settings, select Local Folders and note the path under Local Directory
• Quit Thunderbird completely
• Copy your .mbox file into that directory
• If the folder does not appear later, remove the .mbox extension from the copy, because Thunderbird keeps its own mailboxes as files with no extension
• Start Thunderbird and click the new folder under Local Folders

The case came from a thread on Mozilla Discourse posted after a Thunderbird update. People report imports that hide the folder or bring in only 3 to 5 messages. Some merge the whole mailbox into one email. The copy into Local Folders goes wrong less often than the import dialog, so use it first.

When the count looks wrong, test the file in a second program. If a standalone viewer lists all the messages, the file is fine and the client misread it. If both show one message, open the file in a text editor and check that each message begins with its own From line. A file renamed by hand or cut short during download can lack those lines, and no reader will split it correctly.
Deleting the .msf file next to the mailbox while Thunderbird is closed forces a rebuild of the index. It can fix a folder that suddenly shows the wrong count.

A Takeout export of a busy Gmail account can go over several gigabytes, and an old Thunderbird profile can hold a single Inbox file of that size. One user on Mozilla's support forum described imports of 89 GB and 60 GB mbox files that hung before finishing, while smaller files went through. Notepad is out at that size, and the Gmail-over-IMAP route that some guides suggest is, in the words of a MailsDaddy guide, "not beneficial in the case of large mailboxes."

• Work on a copy stored on a local drive, not on a USB stick or a network share
• Keep free disk space at least equal to the file size, since Thunderbird writes its index next to the mail
• Open the mailbox where you can search by sender or date, instead of scrolling through years of mail

Both sources cover this route: Adam Fowler's post on viewing MBOX files in Windows and the UMaine knowledge base article on Google Takeout mail.
To read old messages only, a conversion is extra work. If the mail has to live in Outlook, you can convert MBOX to PST and open the result as a data file.

Check the extension before you start. A .pst or .ost file is Outlook's own format, and Outlook opens it directly through File > Open & Export.

A Ten Forums thread asks how to read one safely. A mailbox holds other people's addresses and whatever they attached, so privacy matters.

A desktop reader keeps the mailbox off a server. A read-only reader protects the original. Ours opens the .mbox file for reading only, and Thunderbird keeps the messages intact while you only read them, although it adds its .msf index file next to the mailbox and rewrites the file when you delete, move or compact mail.
Demo limits are another concern. Adam Fowler warned back in 2017 that many "free" MBOX tools are demos with a small cap, and vendor guides still put export behind a paid tier.

• The file is from Google Takeout and you need one old receipt or thread. Open it in a viewer and search by sender or date.
• You want to reply to or forward old messages. Put the file in Thunderbird.
• Thunderbird shows one giant message or only part of the mailbox. Cross-check the file in a viewer before you blame the file.
• The mailbox is tens of gigabytes. Use a reader that indexes in pieces and keep the file on a local drive.
• The mail has to sit in Outlook or Microsoft 365. Convert it to PST or import it into the cloud mailbox.
• You want to know whether the file is mail at all. Open a copy in Notepad and look for From lines.

A plain MBOX file opener that only reads is enough for most people who land here with a Takeout export.

• FileInfo: .mbox file extension What the format is and which programs open it
• Google Account Help: download your data Takeout archive types, archive size and split parts
• Mozilla Support: Thunderbird profiles Where Thunderbird stores mail on disk
• Mozilla Discourse: Thunderbird 115.5 mbox files do not read correctly The one-message display case
• WinTips: open MBOX files in Thunderbird The Local Folders copy method with comments from users
• Adam Fowler: viewing MBOX files in Windows Outlook support and demo-limited free tools
• University of Maine IT: Google Takeout mail Which clients open a Takeout MBOX