Mozilla webmail error

I have several domains hosted on DH and had been able to access all
the email accounts via webmail.

Upon upgrading to Debian 7.1 I discovered that I could no longer
access on Iceape & Iceweasel (rebranded Firefox)
browsers, getting following error message on Iceape:

“Proxy Server Refused Connection
The connection was refused when attempting to contact the proxy server
you have configured. Please check your proxy settings and try again.”

Apparently Mozilla browsers have a problem with ipv6 so I disabled
that and had working for several days but it has
reverted to giving error messages again.

I have no problems accessing webmail on other domains, all of which
should be on the same server. And I have no problems accessing webmail
on all domains on Chromium browser.

Obviously this is no major hardship since I am able to access all
webmail using non-Mozilla browsers but I am curious to discover the
source of error and this may be useful to others facing the same