1. View > View Source
  2. Copy the From, To, and CC lines
       (obvious to hackers, less than obvious to many)
  3. Paste into a text editor
  4. Remove the "From", "To", "CC" labels
  5. Add a comma after the From address, and after the last entry in the To list
      (requires understanding of CSV format)
  6. Remove the extra whitespace and newline and the end of every line, making one long line.
  7. Copy the resulting line of comma-separated addresses
  8. Create a new mailing list
  9. In the first slot that asks for a single address, paste in the entire list
      (requires knowing that Tbird will "do the right thing")
 10. Click Ok.
        Wow! It worked. Who knew! 
        (But of course, you're doing it from memory, and you'll have forgotten a step. 
         Just keep repeating until you get it right.)

There HAS to be an easier way. It's a usability issue. This process works, but it's arcane enough that it is really only suitable hackers: 1. View > View Source 2. Copy the From, To, and CC lines (obvious to hackers, less than obvious to many) 3. Paste into a text editor 4. Remove the "From", "To", "CC" labels 5. Add a comma after the From address, and after the last entry in the To list (requires understanding of CSV format) 6. Remove the extra whitespace and newline and the end of every line, making one long line. 7. Copy the resulting line of comma-separated addresses 8. Create a new mailing list 9. In the first slot that asks for a single address, paste in the entire list (requires knowing that Tbird will "do the right thing") 10. Click Ok. Wow! It worked. Who knew! (But of course, you're doing it from memory, and you'll have forgotten a step. Just keep repeating until you get it right.) Anyone smell a usability issue here? The program has been around for quite a while now. (A decade? One-and-a-half? Two?) A usability fix is long overdue! (Thanks for listening.)