Presumably Mozilla Europe is very useful for non-anglophones, but for European English speakers it's useless.
MoFo makes two English versions of Firefox available: “English (British)” and “English”, where the latter is actually United States English (en-US). (As a side note, this should be labelled “English (U.S.)” so it's evident that there's another English version and to be consistent with the other multi-dialect languages.)
The Firefox product page at mozilla.org is smart – it offers British visitors the British English Firefox (in most cases anyway), as do other download points at mozilla.org (such as the front page). Mozilla Europe – which is specifically targetted at Europeans, mind you – offers only the U.S. English version, because the United States is a member of the European Union and is physically located next to the continent of Europe, whilst the United Kingdom isn't.
Wait a sec...