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More information & introduction: http://code.google.com/p/placescleaner/
This extension can be handy to clean up your firefox places database. It will:
1. Delete history entries which have been rarely accessed (default is twice, you can chage the value)
2. Vacuum it, in order to keep the places database clean, compact and efficient.
It also deals with website screenshot data from some buggy versions of Google Toolbar, which will cause your places.sqlite growing extremely bigger. (In some cases, the file size will reach hundreds of megabytes.)