In succession to “improve Firefox performance by defragmenting its database files executing SQLite VACUUM command” tips that required a Firefox restart to execute the command. Mozilla Links have shared another handy tip from Jeremy Orem, that can be done from within Firefox in two short steps:

1. Open Error Console by going to Firefox Tools menu/Error Console
2. In the Code text box paste following single line command:

   Components.classes["@mozilla.org/browser/nav-history-service;1"].getService(Components.interfaces.nsPIPlacesDatabase).DBConnection.executeSimpleSQL("VACUUM");

3. Press Evaluate. All the UI will freeze for a few seconds while databases are VACUUMed