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.

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