Should work ok, don't know for sure, but t'bird is a popular email client and ffox is now the 2nd browser of choice, be odd if spamkiller couldn't work with them installed.
What happens if you double click the message you wish to retrieve rather than using the rescue method? It may work this way.
If you can't get it to work, then mailwasher anti spam may be worth a try, free from click here.
Hope this helps.