Comment 2 for bug 284250

Yes, it is reproducible..
this is the step:
1. Open System -> Network Proxy
2. Set Manual Proxy Config and set your http,ftp,https proxy
3. Now go to Tab Ignored Host, you will see localhost and 127.0.0.0/8 (as default)
4. Open Firefox and enter on URL: http://127.0.0.1/
this will open proxy's webserver which requested using proxy ..... this is wrong because 127.0.0.1 is included in 127.0.0.0/8
5. now open url localhost ..... this will open your webserver or connection refused if there is none
hope this help