These instructions will allow you to open an exported mailbox file (.mbox) in Thunderbird. They assume that Thunderbird is being installed and set up purely to be used as an archive for the exported mailbox.
Note: you will need to know which folder your .mbox file(s) is located in.
Download Thunderbird from this site (link opens in a new tab) and follow the installation instructions
When prompted to provide email information, click Cancel.
Check the box indicating Use Thunderbird without a email account.
Click Exit Setup.
On the main screen, click on the hamburger menu (three stacked horizontal lines) in the top-right corner of the window.
Select New Account > Feed
When prompted, enter a name for the account (name it something appropriate).
Click Finish.
Click on the hamburger menu in the top-right corner of the window.
Select Account Settings and if it is not already selected, select the Feed account you just created.
Under the heading Message Storage, click on the Browse button next to the Local Directory.
Find the folder your .mbox file(s) is located in.
You will be prompted to restart Thunderbird.
When Thunderbird reopens, if you click on the account you just created, under Local Folders a folder containing all emails that were exported to the .mbox file will now be available.