https://bootstrapstudio.io/
I will update with more soon. (Can't remember them.)
Update:
I was going to suggest pixate but they are saying farewell to it :/. It was amazing for prototyping on mobile!!
There was another one as well for wireframing websites etc but I can't remember what it was - and I can't find it. Sorry!
There is also https://www.xamarin.com/ which is awesome but not a fireframework i guess - but really cool still!