On the phone yes, you can get them easily in Iceraven, if you use any of the Firefox for Android forks (Fennec, Mull, etc.) instead, you’d have to add in the extensions list manually to get access to them.
Casting will be unaffected since you’re really just telling a YouTube app running on your Chromecast to play a certain video, some Chromecasts allow you to sideload APKs though, so you could install SmartTubeNext on there