Hi Paul,
I think I can help you with the first question.
Most of the plugins from Felgo are only available/work on iOS and Android devices.
Mostly when they use native libraries/SDKs.
That is the case with Firebase, but also with Chartboost or AdMOb etc
Mostly because Felgo integrates the mobile SDKs there not desktop one(sometimes desktop SDK is not even available).
Some Felgo plugins are available also on desktop, for example Game Network.
Bottom line: 3rd party plugins which are just wrappers to use in Felgo for native SDK: mostly only mobile.
Homemade by Felgo: big chance it also works on desktop, in general everywhere where you can compile it.