This problem isn't specific to Firefox but I'm trying to see how I can use Firefox's debugging capabilities to troubleshoot a network problem. I'm staying in a hotel in China that uses a so-called "captive portal" to authenticate people before using the network. (That means one's first browsing action is redirected to the hotel's web page for entering login info -- as is also commonly done in coffee shops, etc.) Using my own notebook PC, the redirection works if I use wifi, and fails if I use the network cable (Firefox and IE both give the same results). I want my PC to work in both cases (and, in fact, it worked the previous day using a network cable at another location of the same hotel chain, that also uses what appears to be the same redirection scheme). The hotel staff demonstrated to me that a hotel-supplied PC will work with the cable. So from their point of view, something is wrong with my PC, and from my point of view, something is wrong with their network. I need to find out which it is. I enabled HTTP logging in Firefox on my PC. I noticed a cycle of GET requests where a URL was repeatedly URL-encoded (so ':' becomes '%3A', which becomes '%253A', then '%25253A', etc.). So the GET requests keep getting longer until they reach a limit, I guess... the final result being a '400' error ('bad request'). Since their servers are causing the redirections, I can only assume that they have a bug causing this repetitive URL encoding. But, mysteriously, the hotel-supplied PC does not exhibit the problem (and neither does my PC when using wifi). As far as I can tell, Firefox and IE both fail in the failure case, and both succeed in the success case, so I don't suspect the browser. My objective is to either fix my system (if that's where the fault lies) or show the hotel staff that the fault is in their network (by demonstrating that the fault can occur even without my PC being involved). Troubleshooting effort on the hotel-supplied PC is limited by the fact that, once the authentication succeeds, I can't induce it to expire, so I can't experiment much with the redirection mechanism using that PC. Any ideas?