Like i said so many times in this thread, thorn was never an item that was meant to make other carries kill themselves by auto attacking you, how is that a healthy mechanic at all? what's the counter to that?
Thorn was anti sustain, the damage was to mitigate the life steal they got from attacking you, but it didnt do it's job, before they got life steal it would be too strong, after it would be useless.
The new grevious wounds fix that problem, along with an attack speed debuff and a bit of health, the damage return is a bonus but it is not meant to make carries kill themselves, it never was.
After all that is said, no, firoa will not take damage for doing her most basic of basic job which is auto attacking, but she will get reduce healing from all sources - Her ult, life steal, supports, heal spells etc.... no you wont 1v1 her but no one can, you will however reduce her effectivness in teamfights significantly, which is where tanks jobs shines - team fighting.