Comment 114 for bug 113201

I was fortunate enough to be on a tropical island with truly awful Internet access (some satellite, lots of latency, sharing with skype users!, periods of several minutes when DNS servers didn't respond at all according to wireshark) and no proxy server configured. When the ISP's DNS servers decided not to respond, firefox (gutsy beta as of two weeks ago) would do this hanging stuff as well.

This implies that there are other places synchronous DNS lookups are being done which can also hang the interface.

Alternatively maybe it is an extension such as Adblock Plus which I happen to be running everywhere.