I suspect a bad add-on. Since add-ons often are installed externally to Firefox, I suggest starting here:

Open the Windows Control Panel, Uninstall a Program. After the list loads, click the "Installed on" column heading to group the infections, I mean, additions, by date. This can help in smoking out undisclosed bundle items that snuck in with some software you agreed to install. Take out as much trash as possible here.

Then, in Firefox, open the Add-ons page using either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then, if in doubt, disable (or Remove, if possible) unrecognized and unwanted extensions.

Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Does that clear it up?

It also is possible for malware to change your connection settings in Firefox and at the Windows level to redirect your traffic through untrusted servers.

You can check your Firefox connection setting here:

"3-bar" menu button (or Tools menu) > Options > Advanced > Network mini-tab > "Settings" button

The default setting of "Use system proxy settings" should piggyback on your Windows/IE "LAN" settings, but you also can try "No proxy" to see whether that helps.

In this removal guide for cosstminn, the list of programs that you might find in the Control Panel is very long, so I'll just provide a link to it for reference: http://malwaretips.com/blogs/remove-cosstminn-virus/#uninstall (it's #3 under Step 1).