After I followed the instruction by inserting the Text Record 1 that firebase has provided into my NameCheap, this error message keeps popping up:
Current Status: S
I had issues connecting my custom domain as well, apart from using @ in the host field and the CNAME, you'd also need A records. Here's all I ended up with, I sat back a while for the domain to get propagated (I had just purchased it) and waited for 10 minutes after adding all of the records and it worked. Also I've found helpful instructions in this blog post.