When you install a new extension in Firefox, that extension sets up its own preferences in the prefs.js file. Even after you remove that extension from Firefox, some of the preferences are left behind and make the preferences file bulkier with time. A larger prefs.js file can make Firefox load slower and impact the overall performance of the web browser. But now you can use the eCleaner extension for Firefox to get rid of the left behind preference entries from Firefox.
The eCleaner extension for Firefox makes it very easy to locate and remove the junk entries from the prefs.js file of your Firefox profile. This task is very hard to do manually, but eCleaner makes it a matter of pointing and clicking with your mouse. You can download this extension from the Mozilla Firefox add-ons website. The extension installation requires you to restart your Firefox browser.
After eCleaner has been installed, you can acccess it from the extensions page which you can open by pressing the hotkey Ctrl + Shift + A. Alternatively, you can just press the hotkey Alt + P. Then click on the Options button for the eCleaner add-on. This would open the eCleaner window where all the extensions that you ever installed and number of their entries present in the preferences file are listed.
If you want to clear the entries belonging to a particular extension, then just select it from the list and click on the Clear Selected button. This would get rid of all the orphan entries left behind by that extension. Before you proceed to clear the entries, make sure that the extension has been uninstalled from Firefox. If you accidentally clear the entries of an installed extension, then you would lose all the preferences you have set for it.
You can download eCleaner extension for Firefox from https://addons.mozilla.org/en-US/firefox/addon/ecleaner/.