It's 4443, and actually everything appears to be working fine now. I swear this bit me every time I tried to use FF for months...

It must be a different port, otherwise localhost without a port number would work fine. Presumably 8080 or something, which is common for user apps to use since without privileges you can't open a port below 1024 normally.

But what port would firefox connect to, and how would it know?

Chrome doesnt do this either as far as I can tell.

> "Chrome autofills the port"

Sounds like they'd visited the URL in the past and had it in their history.