After 6 months of just “dealing with it” – I honestly thought Firefox did it deliberately to show up badly coded websites – I have finally found a solution.
It is in this thread – http://support.mozilla.com/tiki-view_forum_thread.php?comments_parentId=2926&forumId=1
Seems even the programmers behind Firefox don’t know the reason why – suggestions that it is the Intel onboard graphics at fault, or some deep-lying Windows colour setting – but the answer (for me at least) is this…
Go to Tools->Options->Content->Colours and tick “Use system colours”.
Sorts the problem and websites look as they should again!