Want to clean up the mess that an extension leaves behind after it has been uninstalled? eCleaner is the add-on you need to download and install into Firefox. 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.
Here is a little more about the add-on:
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 :-)
If you want to see more development done on this extension, be sure to let the developer know via his Firefox Add-on page. I think this is a great idea personally and I see a lot of need for something that clears out the mess that some extensions for Firefox leave behind.