Seriously!  from http://kb.mozillazine.org/Compacting_folders

When you delete messages in an email program ("email client") or move them to another folder in the program, they are not yet physically removed - even emptying the Trash does not remove them. Instead, Thunderbird and other email programs simply hide the "deleted" messages and mark them as ready for physical removal. The process of physically removing such no longer visible messages is called "compacting". This means that messages that you think you have deleted are not actually physically removed until you manually or automatically compact the folder they were (are) in.

If you don't compact your mail folders periodically, they can grow very large and make Thunderbird slow to a crawl. If you wait too long before compacting, you can also lose the entire contents of one or more folders. The reason many users have never heard of compacting is that most email programs default to automatically compacting the folder whenever a certain amount of space is wasted, whereas the user has to enable this feature in Thunderbird.

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