Firecrests prefer the comfort of evergreen forest areas, which is the the best place to spot one. Look out for broadleaf forests with an abundance of oak and alder, otherwise you may find Firecrests in habitat among holly (winter) and beech.
On (very) rare occasions they have been known to visit gardens, but you would be lucky for this to happen. In most cases, you’ll will need to actively seek them in the wild. However, according to Wikipedia, “they can thrive in fairly urban areas, provided that suitable habitat is available in parks and gardens.”