Some while ago, I submitted a bug to the Gentoo Bug-trac. I had trouble with Firefox 220.127.116.11 when switching from non-proxy to a proxy-network.
I found that when starting firefox (note: not firefox-bin) the proces wants to connect to www.gentoo.org
Now someone found the issue:
I have the same problem with mozilla-firefox-18.104.22.168.
Firefox tries to connect to the page mentioned in
If DNS or the page itself is unreachable firefox waits until the request
I haven't test it yet... but it might solve the issue...