This has happened to me a couple of times recently, previously refreshing the page in Firefox fixed it, but just now I saw it when checking by downloading the page with curl. Rather than the www.metoffice.gov.uk webpage displaying, I saw the source of the page displayed as text with a spurious "2000" at the start.
2000<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML+RDFa 1.1//EN" "http://www.w3.org/MarkUp/DTD/xhtml-rdfa-2.dtd" > <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en"> <head> <link href="http://fonts.googleapis.com/css?family=Rokkitt|Open+Sans:400,700,400italic" rel="stylesheet" type="text/css" /> <title>Where are the windiest parts of the UK - Met Office</title>
It can't be a Firefox problem since the 2000 was present when I fetched the page with curl. The IP address of www.metoffice.gov.uk (at the time and using Plusnet DNS) was 220.127.116.11.