''preference for keyword.URL works when set, but reverts at start of new session'' I would expect that you are overriding it in with a user.js file in your profile. It that is the case simply remove the override from user.js modify through about:config (not by editing prefs.js) and restart Firefox. user.js is optional, if you don't have the file, its mention does not apply. Another possibility is that the about:config file is write protected from copying from a CD-ROM which is read only. If that is the case you will see several numbered version of prefs.js in your profile.