hi FRHR, you can right-click any preference and reset it to its default value this way.

Yes, but I want to change what Firefox considers default.

To what end?

Default preference values are part of the program code. There's not any (easily) editable file to change.

If your Firefox has a problem retaining customized settings in prefs.js, please see this article for suggestions: How to fix preferences that won't save.

If this is for rolling out Firefox in a business setting, there are some tools to pre-set preferences (but they still don't change the program's internal defaults).

Hello to all,

as jscher2000's said ......Default preference values are part of the program code. There's not any (easily) editable file to change....

see also the option to lock the preference.

thank you

See also http://kb.mozillazine.org/user.js_file as a Preference setting there can override the change made to the Preference in Options/Preferences or about:config after Firefox is closed.

Thank you all, I searched for editing the program code but it was to hard to manage. I asked the question because I edited preferences a lot for privacy and security reasons and realized that alternate preferences in prefs.js and, if existing, user.js slow down Firefox since they are loaded separately from default ones and changed afterwards. I'll try to reduce the number of edited preferences by regular refresh (preferences left over from removed add-ons) and by editing as globally as possible with the smallest number of changed preferences.

It would surprise me if changing prefs itself directly slows down a Firefox startup. More likely the effects of those pref changes slow down Firefox or its startup.

I would anticipate Firefox startup or running may if anything be faster if the pref geo.enabled is false.

If you are security conscious and using Firefox in a none default configuration one thing you may be doing is ensuring History is cleared on closedown. Hopefully any problems with that are resolved, but there have certainly been issues in recent Firefox versions where that option did sometimes cause problems at cloesedown, and possibly a knock-on effect at startup. Some of those problems could have been hangs or crashes and would definitely impact badly on startup times.

I know that is not what you were asking about but if you have problems with startup times maybe it is something to think about. Maybe it is one factor you should change and compare what the before and after results are.