Try to add a comment line at the start of the local-settings.js file as well in case the first line is skipped. Some have reported that this is necessary.

//
pref("general.config.filename", "mozilla.cfg");
pref("general.config.obscure_value", 0);

These functions can be used in the mozilla.cfg file:

defaultPref();	// set new default value
pref();		// set pref, allow changes in current session
lockPref();	// lock pref, disallow changes

The mozilla.cfg and local-settings.js files need to start with a comment line (//).

...and it's lockPref with a capitalised "P" as cor-el already mentioned.

Sorry guys. I was a little sloppy with my post.

In Both of the files the // is the first line All of the directives were exactly lockPref and all ended with a ;

I went through the files so many times checking for punctuation and grammar and then went and posted here without the same diligence. Sorry for that.

Excluding the // which must be the first line, is the order of the directives important, because in my local-prefs.js file, I have

// pref("general.config.obscure_value", 0); pref("general.config.filename", "mozilla.cfg");

Where as cor-el has stipulated

// pref("general.config.filename", "mozilla.cfg"); pref("general.config.obscure_value", 0);

EDIT: Ok, I've tested it on my pc, and it doesn't matter what order I specify those two lines, if I have the obscure directive in the file it responds "Failed to read configuration file"

The local-prefs.js is definitely in the defaults/prefs folder with the channel-prefs.js file (FF is seeing it and racting to it). The mozilla.cfg is definitely in the same folder as firefox.exe (c:\Program Files\Mozilla Firefox)

Did you verify that the mozilla.cfg is a .cfg file and didn't accidentally got a hidden .txt file extension?

A "Failed to read configuration file" error means that Firefox doesn't find the mozilla.cfg or that the file is corrupted. You can test this latter with an empty mozilla.cfg file.

cor-el said

Did you verify that the mozilla.cfg is a .cfg file and didn't accidentally got a hidden .txt file extension?

Yes. I show file extensions and it is definitely just .cfg

A "Failed to read configuration file" error means that Firefox doesn't find the mozilla.cfg or that the file is corrupted. You can test this latter with an empty mozilla.cfg file.

Indeed. Anything more than // in the file results in the failed read.

Is mozilla.cfg a plain text file?

Does this file work if you save it from a Firefox tab as mozilla.cfg ad place it in the main Firefox program folder?

• data:text/plain;charset=utf-8;base64,Ly8KbG9ja1ByZWYoIm1lZGlhLnBlZXJjb25uZWN0aW9uLmVuYWJsZWQiLGZhbHNlKTsK

OMG! Found it (Well, my son did). Seems my eyesight isn't what it used to be (And I'm on a 2540x1440p monitor).. I had a : instead of a ; at the end of a line.

Now, having fixed that, I find that it still does not seem to achieve what I want. I have these lines...

lockPref("browser.search.defaultenginename","DuckDuckGo"); lockPref("browser.search.order.1","DuckDuckGo"); lockPref("browser.search.order.2","Google"); lockPref("browser.search.order.3","Wikipedia"); lockPref("browser.search.showOneOffButtons",false);

The settings in the browser still show as Google. about:config now shows these as the above values, and says they are locked, but I can change the default search engine, I can select/unselect any of the one offs.

Almost there :)

I think that support for setting the search engine via prefs has been dropped to prevent misuse, just like has been done with the new tab setting.

The setting of the current default search engine is stored in search.json.mozlz4 (previously in search-metadata.json) in the Firefox profile folder under the [global] section. Browser Console:

var searchService = Services.search;
var currentEngine = searchService.currentEngine.name;
var engine = searchService.getEngineByName("Google");
searchService.currentEngine = engine; //set current engine to Google

• Bug 1029148 - store "current search engine" configuration outside of prefs (34+)

cor-el said

I think that support for setting the search engine via prefs has been dropped to prevent misuse, just like has been done with the new tab setting.

Meaning what? That one cannot lock down the search engine anymore? Well, that sucks big time.

There doesn't seem to be an extension for this like there is for the newtab issue.

Is there any way to change the contents of that file?

May be I should be looking for a Windows tool that restores not just firefox settings, but other windows settings that people change. Problem with that is that these typically only happen at log on so they can still change settings.