Put that pref into a user.js file.
http://kb.mozillazine.org/User.js_file

Firefox looks at the user.js file every time it starts for additions or changes to preferences, and then writes those to the prefs.js file.

user.js doesn't exist by default, it has to be created. This extension will create that file automatically upon installation and it provides an editing window for the three "user" files from within Firefox. You don't have to dig thru your file system every time you want to do a minor edit.

http://webdesigns.ms11.net/chromeditp.html

Thanks for the info!

You're welcome.

Please click the Solved button next to the answer that solved your Firefox support issue, when you're logged in.