That's an outdated preference. I don't believe it even does anything anymore in the newest versions of Firefox.

Search engine settings aren't stored in your about:config page anymore, mainly because it was too easy for other programs to manipulate the default search engine. It was abused by illegitimate programs that would set the search engine to some malicious website.

Currently, the default search engine can be set using the SearchEngines policies in either Windows Group Policy or a policies.json file, but it can only be done on the Firefox ESR versions.

Hope this helps.

Hi,

I have created policies.json file under installed_directory/distribution. For testing purpose I have used the below in policies.json

{

"policies": {
  "BlockAboutConfig": true
}

}

Note: Regarding verison..under troubleshooting information its "esr"(update channel)

But Still if i type about:config, I can able to do changes..

Are you sure that you have actually created a JSON file? It might seem like a dumb question, but a lot of operating systems (Windows in particular), will add .txt to the file name, making it a TXT file. Essentially, the file name would become policies.json.txt. Since some operating systems, like Windows, often hide the file extension, it's sometimes hard to notice that the file isn't in the correct format.

If that's not it, have you ever installed Firefox's policies using Windows GPO (Group Policy)? If you have, that always had priority over a policies.json file, so it would cause Firefox to ignore that file.

If none of that helps, a good place to check if the Browser Console window right after Firefox opens. It can be accessed by pressing Ctrl + Shift + J on your keyboard or from Tools > Web Developer > Browser Console in the menu bar. This is the place that any errors related to the way that you have configured your policies.json file will be displayed.