I think this could be it. Perhaps I did not explore Thunderbird enough to find the exact match feature. I was surprised because Thuderbird and Seamonkey should be from the same code base, so they should behave similar.
Anyway, thanks for your reply. I might try Thunderbird again after some time. I am very satisfied with Seamonkey, but am afraid that they might scrap the project at any time.
It's similar to what I remember Seamonkey (and and the Mozilla application suite that preceded it) having.
Or are you comparing the quick filter feature in Thunderbird with advanced search in Seamonkey?