toolkit.telemetry.enabled should be false.

The pref should have this setting stored in prefs.js:

user_pref("toolkit.telemetry.prompted", 2);

In case you have rejected it via the notification bar also:

user_pref("toolkit.telemetry.rejected", true);

If you do not keep changes after a restart then see:

• http://kb.mozillazine.org/Preferences_not_saved

I have played with this ad nauseum, with no results. The items you mention can be found set as stated in about:config, but they do not appear in prefs.js, even after clicking "No" on the telemetry prompt and closing Firefox. I'm not getting any error messages stating that the prefs file cannot be modified, and I am able to edit and modify it manually, so the permissions must be set correctly. How now, brown cow?

Try to rename or delete the prefs.js file and possible numbered prefs-##.js files and a possible user.js file to reset all prefs to the default values.

• http://kb.mozillazine.org/Preferences_not_saved
• http://kb.mozillazine.org/Resetting_preferences

Bingo! I renamed prefs.js, re-calibrated Firefox, and it now remembers that I turned off the telemetry, so I don't have that problem any more. One thing I don't understand: there is now an EMPTY prefs.js file, and a file named prefs_temp.js, which is apparently the operative preferences file, since that is where I find the relevant data when I edit the file. Any idea what that's all about?

PS: Thanks very much for the help.

You're welcome.

There should only be a prefs.js file with valid content.
If other (security) software keeps a lock on files or otherwise protect files against changes then Firefox isn't able to update content and you get problems.