User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:44.0) Gecko/20100101 Firefox/44.0 Build ID: 20151111004055 Steps to reproduce: Running 44.0a2 (2015-11-11) on Debian 8. 1. Configure a SOCKS proxy (e.g. ssh -D4567 you@yourserver) 2. Using Firefox manual proxy configuration, SOCKS host 'yourserver', Port 4567, SOCKSv5. 3. Confirm web-facing IP address is the address of 'yourserver.' (Google "what's my ip?" or similar test.) 4. Kill SSH connection/SOCKS proxy. Do not change Firefox proxy configuration. 5. Verify IP address again. Actual results: On step 5, the proxy fails open and you will be able to browse the internet on your local internet connection, unproxied. Expected results: The "proxy server is refusing requests" error page should be displayed, and no connectivity possible.

Thank you for the report, this issue has already been filed as bug 1207798.