The firecrest is, along with its close relative the goldcrest as I am sure you will know, one of the smallest birds to be found in the United Kingdom. It is not generally a breeding species in Dorset although it does breed in small numbers in the south east of England but we are right on the edge of its range here so this is a bird that we mainly see during migration times when birds from northern Europe pass through moving south. Whist it prefers conifers for nesting when on migration it will stop over in scrub and woodland edges of any sort but usually in coastal locations and often in well vegetated valleys so this habitat, coupled with its small size, makes it a bit tricky to find.
The weekly reporting chart shows sightings during most weeks of the year with a few reports during the breeding season and as Dorset has conifer plantations that suit its needs it may be that we have breeding pairs here but that positive proof of breeding has not, as yet anyway, been established. What is evident is the increase in observations in autumn, notably from week 39 until week 49 although records keep coming through until the end of the year. Week 39 is the beginning of October and the peak number of records do seem to come in October and November. It may be interesting to note that there have been far fewer sightings in 2018 than there were in 2017 so weather conditions may have a major impact on their presence in Dorset in autumn.
The distribution map shows just how widespread sightings are but it does tend to back up the premise that they are most often seen in coastal locations but conifer plantations inland do seem to produce records. Portland is one of leading sites for firecrest and this may indicate that birds use Portland as a final feeding stop before setting off out to sea and places further south. Abbotsbury swannery and tropical gardens has its fair share of passage birds too and the car park at RSPB Arne is known for often having firecrest present in autumn but apart from these locations reports are sporadic at best but it shows that you may encounter a firecrest almost anywhere at the right time of year.
If you specifically want to go looking for a firecrest it is difficult to provide any guidance that could be reliable; this is a 'chance upon' species really and one you have to work hard for!