Is this an ordinary web page address, not something special like a media stream or file download? Something may be filtering your requests in the background and modifying how they are identified to Firefox. In case one of your extensions is involved, could you test the same addresses in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: [[Troubleshoot Firefox issues using Safe Mode]]. You can restart Firefox in Safe Mode using Help > Restart with Add-ons Disabled ''(Flash and other plugins still run)'' In the dialog, click "Start in Safe Mode" (''not'' Reset) Any difference?