Like any other database, SQLite database has also become fragmented over time and causes a performance breakdown in Firefox, especially when the web browser is loading and also when the information are take away from the databases.
You can solve the issue by defragging the SQLite database. The Firefox add-on Vacuum Places allows you to defragment (vacuum) the Places database, used by Firefox [You can do this manually. Visit here for information]
The extension consists of a status bar icon, allowing you to vacuum your Places instantly in one click and is very useful for those who surf too much.