Blame the WHATWG for that. They're the reason that v6 addresses in URLs are such a mess. http://[fe90::6329:c59:ad67:4b52%8]:8081/ should work, but doesn't because they refuse to allow a % there. (This is really damned frustrating, because link-locals are excellent for setting up routers or embedded machines, or for recovering from network misconfigurations.)

If it's on the same machine then just use http://[::1]:8081/. Dropping the interface specifier (http://[fe90::6329:c59:ad67:4b52]:8081/) works if the OS picks a default, which some will. curl seems to be happy to work. Or just use one of the non-link-local addresses on the machine, if you have any.

The other frustrating part of this is that it makes it impossible to come up with your own address syntax. An NSS plugin on Linux could implement a custom address format, and it's kind of obvious that the intention behind the URL syntax is that "[" and "]" enter and exit a raw address mode where other URI metacharacters have no special meaning. In general you can't syntax validate the address anyway because you don't know what formats it could be in (including future formats or ones local to a specific machine), so the only sane thing to do is pass the contents verbatim to getaddrinfo() and see if you get an error.

But no, they wrote the spec to only allow a subset of v6 addresses and nothing else.

I very much didn't test it, but this patch might do the job on Firefox (provided there's no code in the UI doing extra validation on top):

  --- a/netwerk/base/nsURLHelper.cpp
  +++ b/netwerk/base/nsURLHelper.cpp
  @@ -928,3 +928,3 @@ bool net_IsValidIPv4Addr(const nsACString& aAddr) {
   bool net_IsValidIPv6Addr(const nsACString& aAddr) {
  -  return mozilla::net::rust_net_is_valid_ipv6_addr(&aAddr);
  +  return true;
   }

An IPv6 literal hostname in a URL must be surrounded by square brackets.