Most middle-aged or elderly widows probably prefer to be addressed in their widowhood in precisely the same way as they would have been addressed while their husbands were alive, ie as Mrs John Debrett. This form of address makes it quite clear that the person in question is either a married woman or a widow. Obviously no single lady or divorcee would be so addressed.
However, it is important to note that very many women who have pursued careers either before or during their married lives, prefer to be addressed by their maiden or professional name. In such cases it is difficult to know whether the prefix Mrs, Miss or Ms should be used. If in doubt, 'Mrs' is probably the wisest choice.
Young-ish widows may prefer to keep their Christian names as a style of address (for example, Mrs Jane Debrett). This is entirely a matter of personal choice, but it can lead someone who is not a close acquaintance to make an incorrect assumption that this indicates that the woman is divorced rather than widowed - an error that might easily cause offence.
Widows of peers were traditionally known as the ‘Dowager Countess of Chesterfield’ but many now prefer the style ‘Hermione, Countess of Chesterfield’ as this may be more widely understood and more practical. Hereditary peers do not use their new titles until after the funeral of their predecessor, so any condolence letter should be addressed to, for example, Viscount Mount rather than the Earl of Maddox.
For a comprehensive guide to addressing people with titles, as well as other correct forms of address, please click here