[sp332]

Is there a practical difference between disabling the features and just ignoring them?

[pc2g4d]

Memory and CPU utilization for starters

[dblohm7]

Firefox makes extensive use of lazy initialization for a lot of its code. You typically don't pay the price for those features unless you actually use them.

[Manishearth]

A button that does an API call consumes very less resources until clicked.