User-Agent: Mozilla/5.0 (Windows NT 6.1; rv:2.0b4) Gecko/20100818 Firefox/4.0b4 Build Identifier: Mozilla/5.0 (Windows NT 6.1; rv:2.0b6pre) Gecko/20100909 Firefox/4.0b6pre At work I have a proxy. Since Firefox 4.0b5 (and following nightlies), if I try to load an https:// page, the throbber moves for a moment but nothing happens, Firefox keeps me on the same page as before. I inspected this behaviour with the Web Console, and noticed that every time FF tries to establish an HTTPS connection, it gets an error 407 from the proxy (something with the terms "ISA", "authentication failed", etc This didn't happen with b4, and with beta5 and following it's all ok if I'm not behind a proxy (e.g. at home). Reproducible: Always Steps to Reproduce: 1. stay behind a proxy (don't know how it's configured though) 2. try to go to gmail, ebay, paypal, etc logins (or pages establishing HTTPS connections) Actual Results: the requested page isn't loaded Expected Results: the requested page should be loaded Don't know if this could have something to do with the recently introduced support for HTTP Strict Transport Security header, but maybe (since the issue is strictly related to HTTPS and STS is, I believe, the major HTTP update that Fx4 beta 5 has got). If it has to do with STS, perhaps this bug should be moved to Core, in this case sorry for the wrong classification.

PS: happens also to a collegue of mine running beta5 as well as the latest nightly on a Win XP (Pro, I suppose) machine.