Junk flags in firefox

[Tom Allison <tallison@tacocat.net>]

I'm noticing that the imap server FLAGS under thunderbird are coming in 
with the flag "Junk".  This is from the client side filtering code that 
Thunderbird has (I assume).

I thought IMAP Flags where more fixed than that, or can any client 
create any Flags within syntactical reason for the IMAP server to store?