ADFS supports several ways of accepting user's credentials. The one with "prompt box" is most likely integrated or basic authentication. It is configurable on ADFS side: http://social.technet.microsoft.com/wiki/contents/articles/1600.ad-fs-2-0-how-to-change-the-local-authentication-type.aspx
If you need to configure forms authentication as a preferred option change in ADFS Web agent web.config order of local authentication types. Set Forms as a the first:
<microsoft.identityServer.web>
<localAuthenticationTypes>
<add name="Forms" page="FormsSignIn.aspx" />
<add name="Integrated" page="auth/integrated/" />
<add name="TlsClient" page="auth/sslclient/" />
<add name="Basic" page="auth/basic/" />
</localAuthenticationTypes>
Regarding the SSO, problem. It depends on browser:
in IE you need to add ADFS URL to Trusted Sites or/and make sure that Trusted Sites zone in IE allows to send windows credentials. See: http://docs.acl.com/ax/310/index.jsp?topic=/com.acl.ax.admin.help/system_administration/t_configuring_internet_explorer_for_integrated_windows_authentication.html
Chrome assumes that any page that contains '.' - dots in address is not secure enough to provide SSO, so probably you'll need to set up a dotless alias for your ADFS host.
in Firefox you need to add ADFS URL network.automatic-ntlm-auth.trusted-uris See: http://sivel.net/2007/05/firefox-ntlm-sso/