In her time of desperation, Queen Firefly needed a strong ally to help Equestria through wartime. Not only that, but a companion to lean on when it seemed she had no one left. So, she arranged a political marriage between herself and the Kirin Queen Hestia.
But it was anything but a happily ever after for them.
They wanted to love each other. But soon enough, Hestia became little more than a sounding board for all of Firefly’s problems. When Hestia spoke up against her, pointing out that she was being selfish and rarely siding with her, Firefly accused her of being controlling and stubborn like someone she once knew. But when asked about this past lover she loathed so much, Firefly refused to share.
Once the King was defeated and the war was over, Hestia found that she had no more reason to stay with Firefly. So she filed for divorce, becoming one more in a long line of hearts that Firefly has broken.