Look out for User Preferences Scripts
Firefox About:config Babylon entries just keeps coming back because they seem to be regenerated from .js scripts. It took me some days to find what was rewriting the Babylon config entries.
First - made sure Firefox was not running or it will replace files on exit.
Looked in (user)\Appdate\Roaming\Mozilla\Firefox\Profiles & in my case \upmc4wvb.default (folder).
This contained several versions of User preference scripts prefs.js, prefs-1.js, prefstemp.js or tempprefs.js
These usually contain Babylon extension setting lines. (need to view in a js editor e.g. Dreamweaver etc.)
Delete or rename these pref.js files with harmless filename extensions. Firefox will replace them as required on first subsequent launch of Firefox. Having done this I finally stopped seeing those Babylon entries in the config file and getting Babylon searches offered to me.