Uninstalling an add-on or extension from your Firefox browser may not always remove its remnants (settings and preferences) from Firefox database. Add-ons store its preferences in Firefox’s SQLite database and these leftovers increase its size, consequently slows down Firefox start-up.
Here is a Firefox extension called eCleaner, which helps you get rid of leftover entries of uninstalled Firefox extensions and speed-up your Firefox start-up.
Here is how to use eCleaner Firefox Add-on
Install the eCleaner add-on from here. Navigate to Tools - Add-on to open Add-ons Manager (or use shortcut key Ctrl + Shift +A).
Locate eCleaner add-on and click the “Options” Button. Select entries that you haven’t installed from the list (use Ctrl + Left mouse click to choose multiple entries) and click “Clear Selected”.