Comment 104 for bug 113201

Scratch the flash theory - I have found the definitive cause.

It occurs if you are using a proxy server. If the proxy server takes a long time to respond to a request then firefox completely hangs. For example right now my ISP is dead (I am using neighbour's wireless to send this). So my internal proxy server takes several minutes to respond to a request as it does dns lookups that ultimately fail and then an error message is returned. The entire firefox interface freezes while waiting on the proxy.

For example I had about 10 tabs open and killed firefox. I then told it to restore my session and it took about 5 minutes before the window even appeared. Each tab was blank and clicking had no effect. It then took about another 5 minutes before tabs slowly started filling with the error content from the proxy server. I still couldn't interact with firefox. I then changed the config to use direct connections and killed/restarted the browser. Firefox still tried and failed to contact the sites, but I could at least use the interface and jump amongst the tabs.

The evidence points to the firefox UI hanging whenever waiting on a proxy connection.