This add-on can be used to clear all those preferences left by extensions after uninstallation, all those garbage which some developers do not care on clearing themselves on their add-ons and which later or sooner will make Firefox to load slower.
To do this, go to the list of add-ons and locate eCleaner, open his Options window and you'll be presented with a list of saved preferences by extension, and their corresponding number of stored values. Select the extensions which you no longer have installed and click the "Clear Selected" button. Volia! garbage erased :-)
You can select multiple extensions at a time, use CTRL+LMB or SHIFT+LMB, you know.
Please note, this is a preliminary first version, with the basic feature of cleaning preferences on a quick way, more advanced features will come later if there is interest from most of you, your feedback and reviews will decide that.