Why web browser don't care about port number when sending cookie?

Question

I have manually created two cookies using firebug:

Cookie1=value1; expires=Sat, 29 Mar 2014 06:21:54 GMT; path=/Ex05Cookie; domain=localhost; HttpOnly
Cookie2=value2; expires=Sat, 29 Mar 2014 06:21:54 GMT; path=/Ex05Cookie; domain=localhost:8080; HttpOnly

The only difference is the domain attribute, one is localhost while the other has a port number.

This is the HTTP header when I try to request http://localhost:8080/Ex05Cookie/:

GET /Ex05Cookie/ HTTP/1.1
Host: localhost:8080
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:20.0) Gecko/20100101 Firefox/20.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Cookie: Cookie1=value1
Connection: keep-alive
Cache-Control: max-age=0

where the header only contains Cookie1 whose domain is localhost.

Why my firefox don't send Cookie2 instead of Cookie1?

Answer 1

This is answered by https://stackoverflow.com/a/4212964/14731 and by the author of RFC 6265:

If we were designing cookies today, we'd definitely make it per-port. However, cookies are widely used on the Internet today and making this sort of change would break too many sites. For the most part, cookies are "done" in the sense that I wouldn't expect them to change much. Unfortunately, that means we stuck with cookies not respecting port number.

Source: https://groups.google.com/d/msg/comp.infosystems.www.misc/6WXdQ2RFhG8/VvOUuR3FNAUJ