Preface: I used Opera for a week while I tried to figure out why FF crashed on me all of a sudden. I also used the built-in email client for Opera, which downloaded all my email to my hard drive.
When I was finally able to fix FF and wanted to go back to FF & Thunderbird for web/email, I was faced with a problem - Thunderbird can only import email from Outlook, Eudora, Communcator, and Outlook Express.
After doing a little digging on Google, and using the old noodle, this is what I came up with, and it worked a treat
===== WARNING: BACKING UP YOUR EXISTING EMAIL (IF YOU HAVE ANY) IS A VERY GOOD IDEA. I CANNOT GUARANTEE THIS WILL WORK FLAWLESSLY FOR YOU =====
Opera Version: 9.02
Thunderbird Version: 1.02
I also want to add that in Opera I had my email filtered by address, which made this very easy for me to import the right emails to the right address within Thunderbird.
In addition, I had only one profile in Thunderbird.
Lastly, I already had the email accounts set up in Thunderbird that I wanted to import emails for before starting this process.
1. For each filter, right-click on the name and select "Export". Save the .mbs file to a folder on your computer.
1a. Copy the .mbs file (Click on the file and then Ctrl+C) [This part is optional but made things go faster for me]
2. Navigate to C:/Documents & Settings /<user name>/Application Data/Thunderbird/Profiles/<profile name>/Mail (where <user name> is the username for your windows login, and <profile name> is the profile name in Thunderbird)
3. Find the email account folder you're importing to, eg mail.something.com and go to the "Inbox" folder
4. Create a folder with .sbd as the suffix (eg, importedmails.sbd)
5. [Copy/]Paste the .mbs file and rename to filename.msf (eg, importedmails.msf)
6. [Copy/]Paste the .mbs file again and rename to filename (no file extension, eg importedmails)
It's important that in steps 4,5, and 6 you use the same name for all 3. So if you start with funstuff.mbs, you should have funstuff.msf, funstuff.sbd, and funstuff in the same folder when you're done.
I hope this helps someone else looking to import their emails from Opera to Thunderbird