The Earth travels around the sun in an elliptical pattern, but it is still fairly close to a circular orbit.
The seasons on Earth are determined by the angle of sunlight that hits the planet according to the axis of rotation of the planet, not the proximity of the planet to the Sun.
A comet might heat up when closer to the Sun on an ellipse with a high eccentricity (deviation from perfect circle), but the Earth’s orbit does not have a high eccentricity.
Also, if you know part of an elliptical path you can figure out the period and axes of the ellipse.
It can thus be calculated that Halley’s comet actually does not go out as far from the Sun as Pluto.