So you want to peek under the hood of all those mysterious Firefox 3.x databases? Easy does it: just install SQLite Manager in… Firefox itself (for easy installation visit SQLite Manager on the AMO site).
This add-on can be activated from the Tool menu and opens in a separate window. By default, a shortcut to your profile directory is provided, but there is nothing preventing you from opening other sqlite3 databases when you’re done staring at the places.sqlite database.
On Mac OSX there is lots to explore about your Mail.app settings in your ~/Library/Mail directory, e.g to optimize mail performance. Just be careful that you don’t make any changes on the live database (you work on a back-up copy, right?).