Well, first I suppose you are talking about Thunderbird and not Firefox...

Then I guess you run a Pop account, and so when you delete your mails in Thunderbird, they remain unchanged on your mail server.

For an unknown reason, Thunderbird forget that he already deleted all these old mails and download them another time.

So you have two solutions:

• switch to an imap account (more efficient than Pop)
• set up your pop account in order that deleted mail in Thunderbird are also deleted on the server

Thanks for the help. Your "suppose" was right. I meant Thunderbird, but since I am a Firefox user, I unfortunately typed Firefox...