The page linked by
acid_kewpie has a list of the ports used for
Yahoo! connections. You can test whether your SNAT firewall is allowing said connections by using a tool such as
Firewalk. You can also test Squid itself by appending port numbers to URLs and observing whether an
Access Denied error message is generated (for example, to test port 5050 you could try to access
http://www.linuxquestions.org:5050). That said, if I was in your position I would first try to verify whether all the required
Yahoo! features will work with a proxy server in the first place, as it wouldn't be surprising if they require SNAT to work properly. FWIW, the page linked by
acid_kewpie includes warnings about this sort of thing.
In any case, here's an example Squid ACL to allow the ports on that page:
Code:
acl yahoo_stuff port 80 5000 5001 5050 5100 5101
http_access allow yahoo_stuff