Multi-Folders and Multi-MSF files

I installed TB and created an IAMP account in TB yesterday.
I noticed that there are multi-folders in the directory:
E:\...\ThunderbirdPortable\Data\profile\ImapMail\
Here is the folder names:
imap.mail.yahoo.com
imap.mail.yahoo-1.com
imap.mail.yahoo-2.com
imap.mail.yahoo-3.com
imap.mail.yahoo-4.com
imap.mail.yahoo-5.com
imap.mail.yahoo-6.com
imap.mail.yahoo-6.com.sbd

Do I have to keep all these folders?

I also noticed that every time I turn on TB, TB creates a new "INBOX-X.msf" file (where "-X" is a number) in the "imap.mail.yahoo-6.com" folder.
And now there are 10 copies of the "INBOX-X.msf" files in the folder.
INBOX-1.msf
INBOX-2.msf
...
INBOX-8.msf
INBOX-9.msf
INBOX-10.msf

What is wrong?

Your helps are much appreciated.