[pcwalton]

Don't use a fork and don't customize your user.js. It is exceedingly unlikely that excess power consumption is caused by anything you can easily tweak. Instead report issues to Mozilla.

[PrimeMcFly]

Very bad advice.

Some forks have clear advantages, are well maintained and trustworthy.

There are numerous reasons to maintain a user.js file also, not just performance reasons.

Indeed, report issues to Mozilla directly, but that isn't inconsistent with using a fork and maintaining a user.js