Here is a way to use Thunderbird to examine an mbox file. If you have an mbox file from a UNIX machine, or you exported it from another system such as the Google export API, it is often easiest to use a mail reader to examine and sort the mailbox, especially if you need to search messages or work with attachments quickly.
For Thunderbird 10.0.X for Windows, there is currently a dearth of plug-ins and extensions. Also, there is no native way to “import” an mbox file. But if you place the mbox file in the “Local Folders” directory, it will be indexed by Thunderbird and show up as a Folder.
If you already have Thunderbird installed, start with step 3.
1. Download, launch Thunderbird
http://www.mozilla.org/en-US/thunderbird/
On first launch, it might say “Use Thunderbird as the default client for:”, but uncheck “E-Mail”, “Allow Spotlight to search messages”, “Always perform this check when starting Thunderbird”, then click “Skip Integration”.
2. Mail Account Setup
You need to let it create a Thunderbird account so that you have the directory structure to place the mbox file in the right location. The easiest way I found to do this is to set it up with a fake gmail account, or your own. Just don’t give it the password. ([email protected]?) Then “Create Account” to make the directory structure.
(Thunderbird 31)
Would you like a new email address?
unselect “ugh.org”
“I think I’ll configure my account later”
Accounts -> Create a new account:
click on “Newsgroups”
Identity:
Your Name: First Last
Email Address: [email protected]
Incoming Server:
Newsgroup Server: blah.ugh.org
Account Name: NEWS-UNUSED
DONE
3. Find your Thunderbird “Local Folders” Directory
Now you can click on “Local Folders” and then “View Settings for this account” to see where it is looking for local mail folders. Something like this:
C:\Users\myself\AppData\Roaming\Thunderbird\Profiles\syg8c7w1.default\Mail\Local Folders
(You need to have allowed “Show hidden files, folders, and drives)
or for Mac
/Users/username/Library/Thunderbird/Profiles/k994zou
4. Drop mbox into Local Folders
Quit thunderbird. Navigate to your “Local Folders” directory in an explorer and drag and drop your mbox file in there. You might change the name to something short, like user-mbox0 or as appropriate.
5. Browse mbox
Restart thunderbird, and you should see the mailbox in your Local Folders list
SOURCE: https://commons.lbl.gov/display/[email protected]/Reading+an+mbox+file+with+Thunderbird