The Mishnah taught: She acquires herself through a get.

As the pasuk states (Devarim 24, 1), "...and he wrote for her a severance document (and she went and she was with another husband)''.

The Mishnah also taught: ...or through the death of the husband.

As the pasuk states (ibid), "...or if the latter husband shall die''.

Question: This only teaches about the death of the latter husband; what about the death of the first one?

Source #1: If for the latter one, whose release is not widespread (as she does not become permitted to her ex-husband), his death permits her; the first husband whose release is widespread (as she can now marry anyone) should certainly permit her!

Source #2 (R. Chuna): There is a pasuk that teaches that death permits her, as the pasuk states (Devarim 25, 5), "When two brothers will exist together and one of them dies and he has no child (his widow will perform yibum with his brother) - but if he had a child, his death permits her.

Source #3 (R. Yosi bei R. Bun): If death does not permit a married woman, why would the Torah need to forbid the cases of a widow marrying a Kohen Gadol and a divorcee or chalutzah (widow who performed chalitzah instead of levirate marriage) marrying a regular Kohen?

Rejection (R. Yochanan bar Maraya): Perhaps the Torah only permits yibum after death (but otherwise, death would not permit her to marry).