I'm not sure if this is a migration problem or not. I have some mail filters that use the Reply-To: header in 4.7.x. When importing into Seamonkey, the rules are maintained even though filtering on Reply-To isn't supported. The net effect is that when you look at the rule in the Message Filters dialog, the rules are empty. But when you look at the rules.dat file, the rule is still there.
We don't support custom headers in 6.0. When a 4.x filter is migrated containing something which hasn't made it to the new release, the ui doesn't know how to treat it, hence the blank reading for your Reply-to header filter.
Since alec is leaving the group, don't know who's really going to inherit filter ui stuff in future releases. Assigning bienvenu, cc putterman for a stab at it.
I guess this is really just another duplicate of bug 16902... *** This bug has been marked as a duplicate of 16902 ***