Some while ago, I submitted a bug to the Gentoo Bug-trac. I had trouble with Firefox 18.104.22.168 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-22.214.171.124.
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...