The Enterprise Policy Generator works, that's sure. The reason is simple: The Enterprise Policy Generator only generates the JSON content, it's not possible that something goes wrong in this step. ;-) If a policy does not work then either the content of the policy is wrong or there is a bug in Firefox. Or you didn't put the file into the correct directory.

If you open a GitHub issue with steps to reproduce your problem then I'll have a look.

By the way: You can use the Enterprise Policy Generator to disable Firefox updates, but really, you shouldn't do that. There is nothing more important than to update your browser for security reasons.