In addition to checking your security software to see if it's interfering, you can check some things to make sure you're not infected with anything.
Goto "Internet Options" in Windows. On the "Connections" tab, click "LAN settings". Make sure "Automatically detect settings", "Use automatic configuratio script" and "Use a proxy server..." are all unchecked.
In Windows Explorer, goto "C:\Windows\System32\drivers\etc", double-left-click on the "hosts" file and choose to open it with Notepad. Make sure that there are no non-blank lines that do not begin with '#'. As in, make sure there are no active entries (except for
127.0.0.1 localhost if you're on WinXP).
In Windows, goto "Network and Sharing Center" and click on "Change adapter settings". Then, right-click on the adapter you're currently using and click "properties". Under the "Networking" tab in properties, double-left-click on TCP/IPv6 and make sure "Obtain automatically" is set for both IP and DNS. Repeat for TCPIPv4.
Download the free version of Malwarebytes (when/after installing, choose to use the 14-day premium trial) and scan your computer with it.
Do the same with Adwcleaner just for good measure.
You can uninstall Adwcleaner and Malwarebytes when you're done with them if you want.
In Firefox, goto the URL
about:preferences#general and click on "Network Settings" at the bottom (or search for it). in there, set "No proxy" and ok out. Also, make sure you have no extensions in
about:addons. Then, see if that helps things work in Firefox. If it does, then it's probably an issue with the Internet Options proxy setting.