From Firefox 52, the battery API has been disabled and is only available to chrome/privileged code (I would assume add-ons fall into this category). For earlier versions, the battery API is enabled by default. It can be disabled by going to to
about:config and setting the
false. Changes take effect immediately on subsequent page loads.
Chrome & Android
Both the mobile and desktop version are affected. This feature is enabled by default and there doesn't seem to be any way to disable it yet, though you shouldn't be using Chrome at all if you care about privacy.
The Android browser is affected since version 40, without any way to disable it either.
The feature is under consideration and isn't (yet?) implemented in any current versions.
This feature isn't currently implemented in any versions, neither desktop nor mobile.
Opera on desktop is affected, the feature is enabled without any way to disable it. Opera Mini isn't affected.
Here's a demo you can use to check whether you're vulnerable. There is also Can I use which provides fairly up to date information about which features are supported in browsers. I'll do my best to keep this answer up to date but I suggest you double check on there just in case something changed.