User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:53.0) Gecko/20100101 Firefox/53.0 Build ID: 20170413192749 Steps to reproduce: Just navigate with Firefox for some time (about one hour) I'm using proxy.pac of my Company to connect to Internet Actual results: Suddenly network is no more working: to restore it I need to restart Firefox. Other application are working fine (so I exclude a proxy issue)

Just a note: before upgrading to Firefox 53 I've never experienced this issue

We used Firefox ESR 45.x and do have the same problem after an upgrade to 52.1.0. We also use a proxy.pac File and we did not have this issue before. After a while of browsing Firefox stops loading pages until it is restarted.

Can you please provide an HTTP log (https://developer.mozilla.org/en-US/docs/Mozilla/Debugging/HTTP_logging) during a reproduction for further investigation? Thanks.

Hello, today it took me a while to reproduce the issue: I've just discovered that I was able to reproduce it some minutes after having launched a Private Browsing session. So I'm pretty sure that the issue is related to Private Browsing mode, since I usually have a private section opened alongside normal one. Note: in the logs I've just renamed my company URLs...

Today I've experienced the issue without any Private Session active, I'm attaching new logs, please let me know if you need additional data to understand the root cause of the issue :) BR, Alessio

Alessio - one other question while digging into this. Do you happen to know if your company proxy uses NTLM authentication?

Hello Nicholas, my company proxy should use NTLM authentication, but I'm still trying to get a confirmation from IT...is there a way to manually verify which authentication is used? Maybe from Firefox network inspector?

Here's how you can verify if your proxy uses NTLM: Go to Developer Tools (F12), change to the Network tab. Force-reload your page (Ctrl-F5). If there is a red 407 line, click it. Look at the response headers. If your proxy uses NTLM there is one that says "Proxy-Authenticate: NTLM".

Thank you for the suggestion, let me paste request and response: GET /?gws_rd=ssl HTTP/1.1 Host: www.google.it User-Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:53.0) Gecko/20100101 Firefox/53.0 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Language: it-IT,it;q=0.8,en-US;q=0.5,en;q=0.3 Accept-Encoding: gzip, deflate, br Cookie: NID=102=MLiWmNrF8rYS_V6ScmfUY08yHUsEdvOGLih_bygoC9xThMYZezV4ZDPjyzhg206bfEdac3-hDItOY7WU5YfcJe2e8ZAydp0X-63Z7R3-VgBleA44PKv0ob6yzpVuz4ROp2g35w; CONSENT=YES+IT.it+V8; OGPC=5061821-6:; OGP=-5061821: DNT: 1 Connection: keep-alive Upgrade-Insecure-Requests: 1 Pragma: no-cache Cache-Control: no-cache HTTP/1.1 407 authenticationrequired Via: 1.1 10.12.9.174 (McAfee Web Gateway 7.6.1.2.0.21212) Date: Wed, 03 May 2017 06:53:44 GMT Content-Type: text/html Cache-Control: no-cache Content-Length: 4113 Proxy-Connection: Keep-Alive Proxy-Authenticate: Negotiate Basic realm="McAfee Web Gateway" Following requests contain: Proxy-Authorization: "Negotiate YIIJwQYGKw..."

As mentioned above, we do have the same problem. I hope i can contribute something to find the issue :-). We use Proxy-Authenticate:"NegotiateNTLM" but i think we use kerberos, too.