I feel Ruan Mei would still be the OP pick with her and suits her so much more because of the BE buffs, weakness break efficiency, sp generation etc
100% people talk a lot a out meeting the break effect thresholds on firefly but atk thresholds will pose a similar issue.
It works because Robin's and fireflys enhanced states have the same duration if you proc them one after the other.
Robin also gives a strong dmg% bonus which is really good for firefly, especially if your not running her with Ruan Mei.
While she is probably not BIS, they definitley have some synergy.