Question, Why was your value true?
The default is actually false which is what you recommend telling people to set.
If you havent been mucking with it, then I can only assume that its been wrongly changed, or at one point firefox decided to change the default from true to false, and your browsers didnt change.
When I installed firefox just now it was already on false.
To confirm, you never modified this value before?
If anyone has a firefox version, or can make one that doesnt work, without manually changing that value information would be good.