Also the new Version of Thunderbird has vastly improved search, I think.
1 - Message -> Tag -> New Tag... call it 'Read' and choose a color
2 - Tools -> Message Filters.. click 'New' and give the filter a name (eg. color read messages)
3 - select 'Status' - 'is' - 'Read' from the first row of drop down menus ('For incoming messages...')
4 - select 'Tag Message' - 'Read' (under: 'Perform these actions:')
Edit: ah sorry, just saw that it is now an built-in option for the Thunderbird message filters.
Edit2: even better, just saw that the spam filter now has an option for "don't mark as spam if sender is in address book" :-)
... how does this guy get anything accomplished?
Also the new Version of Thunderbird has vastly improved search, I think.