Does the ceramic mug keep coffee hot for long?

Not as long as an insulated stainless tumbler. Ceramic holds heat for roughly the length of a commute or a meeting, so drink within an hour or two.

Is the mug leakproof?

It is described as leak-resistant. The lid reduces splashes in a bag but is not sealed against tipping or shaking.

Is the mug dishwasher and microwave safe?

Treat it as hand-wash and keep it out of the microwave unless the mug itself says otherwise. Ceramic with a lid and any metal trim does not belong in either.

Are the earmuffs one size?

Yes, they are a regular-fit faux pair with no sizing. They suit most adults but fit is not adjustable for smaller heads.

Is this a good gift for a coffee drinker?

It works as a light seasonal gift, especially for someone who dislikes metal-lined mugs. A daily commuter is better served by an insulated steel tumbler.