Voltage sage of 0.2V on 1S at hover is very common. However, the deeper sag may be a concern. So, yeah, get a few good batteries, like the GNB I mentioned previously, and compare those.

It looks like the Firefly comes with a GNB27 connector. Finding the right GNB 450mAh with GNB27 connector in stock has been somewhat of a pain. I only found them on Amazon, and they have a little bit of a mark up:

https://www.amazon.com/GAONENG-Battery-450mAh-GNB27-Connector/dp/B097SJBMVB/