Linux/Mac/windows: In Optimize Firefox Databases With a Single Line of Code you were asked to copy and paste a line of code in the Error Console command to vacuum your fragmented SQLite databases.
Now you can get this done automatically with the Improved Vacuum Places add-on. This experimental add-on for Firefox 3.5 and higher vacuums your Places Database each time you click on the small icon it places on your Firefox status bar.
Right-click on that icon and click-on “Preferences”. In the “Vacuum Places Improved Settings” windows tick the “Automatically clean places” checkbox. Now your Places databases will be automatically cleaned when the number of Firefox starts setted reaches its maximum.
Improved Vacuum Places is a free download.