When installing a Firefox add-on it stores its preferences in Firefox’s SQLite database. Even if you have removed the add-on, its entries may not always completely remove from Firefox’s database. It might be a big big problem if you try lot of add-ons. It increases the size of SQLite database which will adversely affects Firefox’s startup, page loading times and stability.
Previously we published a tutorial on how to remove add-on leftovers from Firefox manually. That was a little difficult process for some users, as they want to identify each add-on’s entries manually. Here is an add-on called “eCleaner” to make things a lot easier.
eCleaner is designed to quickly and easily remove preferences left by extensions after uninstallation and help you speed-up your Firefox start-up.
Install the eCleaner add-on from Firefox’s add-on repository. After installation, restart Firefox and 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 add-on entries that you haven’t installed from the list (use Ctrl + Left mouse click to choose multiple entries) and click “Clear Selected”.(Source)