Here is an obfuscated version of mozilla.cfg with the above posted content.

Copy and paste the full code in the location bar and save the file as mozilla.cfg
Copy the file to the main Firefox program folder where the Firefox.exe program resides.

As this file is obfuscated you only need a line to specify the mozilla.cfg file in local-settings.js

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

data:application/octet-stream;base64,
PDwaF3l8cHhdf3JzNS9vf3yEgHJ/O4CBbn+Bgn07dXx6cn1udHIvOS0vdYGBfUc8PISEhDuAfHpy
hHJvfW50cjtwfHovNkgaF3l8cHhdf3JzNS9vf3yEgHJ/O4B1cnl5O3B1cnB4UXJzboJ5gU9/fISA
cn8vOS1zbnmAcjZIGhd5fHB4XX9yczUvbn19O4J9cW6Bcjtye25veXJxLzktc255gHI2SBoXeXxw
eF1/cnM1L29/fISAcn87cXyEe3l8bnE7goByUXyEe3l8bnFRdn8vOS1zbnmAcjZIGhc=

I found my problem: I did store the mozilla.cfg with the defaults. However it should be on top of the Firefox-Programfolder in Windows e.g. c:\program files\mozilla firefox

Your last post gave me the hint. Now it works without obfuscations as well. Thanks!

I am having the same issue with locking the setting. If you look at the following support thread:

https://support.mozilla.com/en-US/questions/853068#answer-218572

I have tried a lot of "fixes" I have seen on other forums and still no luck. Has anyone actually go this to work on firefox 5?