The firecrest is not a subspecies of the goldcrest; they are separate species in the same genus. The goldcrest has the scientific name Regulus regulus; the firecrest is Regulus ignicapilla.
According to Wikipedia, "The common firecrest was first formally described by Dutch zoologist Coenraad Jacob Temminck in 1820 as Sylvia ignicapilla; the relatively late identification of this common European bird arose from a perception that it was just a variety of the goldcrest."
The RSPB gives the length and weight of the firecrest as 9 cm and 5 to 7 grams, and the goldcrest as 9 cm and 6 grams – so no difference at all really. It tells us that the firecrest "vies with the goldcrest for the title of the UK's smallest bird."
However ... the RSPB also gives the breeding population of the firecrest as only "550 territories", compared to 610,000 for the goldcrest. Wikipedia classifies the firecrest as a winter visitor to Britain (the south of England and the south coast of Wales), meaning that it doesn't breed here and so it's debatable whether it's actually native to Britain. You might hesitate to take Wikipedia's word over that of the RSPB, but there are about 1,109 goldcrests for every firecrest; and despite what Wikipedia itself says ("common European bird" – see above), I wouldn't describe even the goldcrest as particularly common. (For comparison, there are 5.1 million breeding pairs of blackbirds, and another five million winter here.)
Even so ... it would be harsh not to allow the firecrest as an alternative answer to the question.
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