The reason behind this Firefox speed up tricks is that Firefox is using SQLite database to store it’s info (eg. Bookmarks, cookies, histories, etc). After sometime of using it, data will be fragmented and the database will grow bigger and it will affect the speed of Firefox. To solve the fragmented issue, we can perform a ‘vacuum’ command to the database. * This might be an old trick but work!

To speed up Firefox using Vaccum, follow the steps below:-

• Start your Firefox, and Go to ‘Tools’ -> ‘Error Console’
• Paste the following text in the “Code:” text-box and press “Evaluate”

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

• Your Firefox will freeze a while when these Vacuum operation is running
• Done, your firefox should run faster now. (* this operation doesnt affect if your firefox is freshly install)

Credit: Oremj from Mozilla