From MozillaZine Knowledge Base
Opera's *.mbs files are actually mbox files, the same format that Thunderbird uses. They just use a different file name extension. If you're using a POP account you need to create, convert and import them as described below. If you're using a IMAP account the messages are stored in remote folders, you could just create the same IMAP account in Thunderbird and reuse the existing folders.
- In Opera right click on the folder you want to export, select export, and press the Save button. This should create a mbox file with the folders name and a .mbs file extension
- Repeat for the rest of the folders you want to export.
- Use this web site to export the Opera address book as a .csv file. If you are running Windows you could use Dawn to convert the file used to store the address book to a .csv file instead. Look at "Opera Directory" in the Paths section of Help -> About Opera. The *.adr file (address book) is somewhere in that directory tree.
- Exit Opera.
- Rename each of the *.mbs files to have no file extension.
- If you don't have the ImportExportTools extension install it. If you have problems installing it see this article.
- Start Thunderbird and use Tools -> Import/Export in mbox/eml format -> Import mbox file to import all of the mbox files. This will import the Opera mail folders.
- Use Tools -> Import -> Address books -> Text files to import the .csv file. This will import the Opera address book.