mbox-reader is a web-based tool that converts .mbox files into a compressed ZIP archive of .eml files, which can then be opened in most email reader applications.
If the .mbox file is large (100 MB or more), this tool can still process it directly in your web browser. However, it will export multiple ZIP files in that case, so be sure to allow multiple file downloads if your browser prompts you.
This tool has been tested with .mbox files exported from Gmail.