Thunderbird can read mbox files well.
You can drop your mbox files to the Local Folders directory of Thunderbird and restart Thunderbird. It will index the emails and show them.
To find the Local Folders directory of Thunderbird:
Open "Account Settings" and click on "Local Folders" to see where Thunderbird is looking for local mail folders.
It may be something like:
"C:\Users\myself\AppData\Roaming\Thunderbird\Profiles\8c7wa8d7y1.default\Mail\Local Folders" on Windows.
"/home/eric/.thunderbird/3d1ss5n8.default/Mail/Local Folders" on Linux.