Mozilla Links blog has a quick hack for improving Firefox performance by defragmenting its database files executing SQLite VACUUM command

• Open the Error Console: Tools menu/Error Console
• In the Code text box paste this (it’s a single line):

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

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