Read .msf/mbox files offline in Thunderbird
The mozillaZine article Importing folders may contain a possible answer.
It dates from July 2011, but may still pertain.
The steps it describes are the following :
- Create a new Thunderbird profile without any installed extensions
- Download and install the ImportExportTools extension, restart Thunderbird
- Use the menu
Tools -> ImportExportTools -> import mbox file - Browse to the mbox file and press the Open button. The imported inbox may have a suffix appended to its name for uniqueness.
A second possible procedure is described in the mozillaZine article Manually importing and exporting, which is basically replacing an existing inbox file with the old one. For that to work you should probably use the same version of Thunderbird that created the mbox.
Like Stephanie said - start off by closing Thunderbird and backing up your profile folder. Since you said that you're using Windows 7, it should be located in "C:\Users\\AppData\Roaming\Thunderbird\Profiles"
Once your profile has been backed up to a safe location you can start to make the changes.
Within your profile folder, you should see two folder: "ImapMail" and "Mail" These are where your IMAP mbox files and your POP/Local folders are kept (respectively).
Go into the "ImapMail" folder and find the folder for the Gmail account that you want the data from. Select the mbox files (ignore the .msf files - these index files will be regenerated the first time you go into the folders again) and copy them into the "Mail\Local Folders" directory in your profile. You can change the filenames if they conflict and they'll show up as the new filenames when you restart Thunderbird.
Restart Thunderbird and you should see your old folders.