Hi,
When I first saw this feature something like 4 years ago, I thought that it was the only thing missing in Firestorm. But for some reasons it never made it way in FS.
So finally, I decided to borrow some code from MK and put it in Firestorm.
https://github.com/mya-ansome/phoenix-firestorm-rlv/releases/tag/Firestorm_RLV_6.4.12_v0.2_alpha
I hope that it will work for you and that you will have fun with it.
Take Care
Mya