1. What? How is a fleshed out API dedicated to presenting websites "less general"?
2. Are you talking about Custom Tabs or Safari Services? Because Safari Services handles deep links fine...
3. Custom tabs work with Firefox if enabled in Settings
I mean Custom Tabs, like most of Google's efforts on Android start, is a half-baked concept. And like most of Google's efforts on Android, there is a chance that they will actually flesh it out, and there's a chance they might not and it'll just kinda exist and undergo death by a thousand small-but-unaddressed bugs...
SFSafariViewController had its teething issues back in the day but Apple worked on it until it became something something most apps use, most people use, and it's seamless enough they don't know they're using it (especially because it supports everything Safari does, from ad blocking to reader mode)